Treatment of acid gases using molten alkali metal borates, and associated methods of separation

ABSTRACT

The removal of acid gases (e.g., non-carbon dioxide acid gases) using sorbents that include salts in molten form, and related systems and methods, are generally described.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/090,180, filed Nov. 5, 2020, and entitled “Treatment of Acid GasesUsing Molten Alkali Metal Borates and Associated Methods of Separation,”which claims priority under 35 U.S.C. § 119(e) to U.S. ProvisionalPatent Application No. 62/971,488, filed Feb. 7, 2020, and entitled“Treatment of Acid Gases Using Molten Alkali Metal Borates, andAssociated Methods of Separation,” and U.S. Provisional Application No.62/932,410, filed Nov. 7, 2019, and entitled “Process for RegeneratingSorbents at High Temperatures,” each of which is incorporated herein byreference in its entirety for all purposes.

TECHNICAL FIELD

The removal of acid gases that are not carbon dioxide using sorbentsthat include salts in molten form, and related systems and methods, aregenerally described.

SUMMARY

The removal of acid gases that are not carbon dioxide using sorbentsthat include salts in molten form, and related systems and methods, aregenerally described. The subject matter of the present inventioninvolves, in some cases, interrelated products, alternative solutions toa particular problem, and/or a plurality of different uses of one ormore systems and/or articles.

Certain aspects are related to methods. In some embodiments, the methodcomprises exposing a sorbent, the sorbent comprising a salt in moltenform, to an environment containing a non-CO₂ acid gas such that at leasta portion of the non-CO₂ acid gas interacts with the sorbent and atleast a portion of the non-CO₂ acid gas is removed from the environment.

Other advantages and novel features of the present invention will becomeapparent from the following detailed description of various non-limitingembodiments of the invention when considered in conjunction with theaccompanying figures. In cases where the present specification and adocument incorporated by reference include conflicting and/orinconsistent disclosure, the present specification shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described byway of example with reference to the accompanying figures, which areschematic and are not intended to be drawn to scale. In the figures,each identical or nearly identical component illustrated is typicallyrepresented by a single numeral. For purposes of clarity, not everycomponent is labeled in every figure, nor is every component of eachembodiment of the invention shown where illustration is not necessary toallow those of ordinary skill in the art to understand the invention.

FIG. 1 is, in accordance with certain embodiments, a schematic diagramof a sorbent being exposed to an environment containing non-carbondioxide acid gas.

FIG. 2 is, in accordance with certain embodiments, a schematic diagramof a sorbent being exposed to an environment containing non-carbondioxide acid gas that is part of and/or derived from the output of acombustion process.

FIGS. 3A-3D show, in accordance with some embodiments, the dependence ofacid gas concentration on loading for Na_(x)B_(1-x)O_(1.5-x) (x=0.75) at(A) 600° C., and (B) 700° C., and for(Li_(0.5)Na_(0.5))_(x)B_(1-x)O_(1.5-x) (x=0.75) at (C) 600° C., and (D)700° C.

FIGS. 4A-4D show, in accordance with some embodiments, uptake,displacement, and release of acid gas mixtures forNa_(x)B_(1-x)O_(1.5-x) (x=0.75) at (A) 600° C., and (B) 700° C., and for(Li_(0.5)Na_(0.5))_(x)B_(1-x)O_(1.5-x) (x=0.75) at (C) 600° C., and (D)700° C.

FIGS. 5A-5D show, in accordance with some embodiments, forNa_(x)B_(1-x)O_(1.5-x) (x=0.75) (A) SO₂ capacity under 5° C./mintemperature ramp, isothermal uptake, and predicted by Equations 3 & 4(B) XRD at 25° C. after reaction with SO₂ at 600° C. and 700° C., for(Li_(0.5)Na_(0.5))_(x)B_(1-x)O_(1.5-x) (x=0.75) (C) SO₂ capacity under5° C./min temperature ramp, isothermal uptake, and predicted by(analogous) Equations 3 & 4, and (D) XRD after reaction with SO₂ at 25°C., 600° C., and 700° C.

FIGS. 6A-6B show, in accordance with some embodiments, (A) H₂S capacityfor Na_(x)B_(1-x)O_(1.5-x) (x=0.75) under 5° C./min temperature ramp,and predicted by Equations 8 & 9 (B) XRD at 25° C. after reaction withH₂S at 600° C. for Na_(x)B_(1-x)O_(1.5-x) (x=0.75) and(Li_(0.5)Na_(0.5))_(z)B_(1-x)O_(1.5-x) (x=0.75).

FIGS. 7A-7D show, in accordance with some embodiments, (A) NO₂ capacityfor Na_(x)B_(1-x)O_(1.5-x) (x=0.75) under 5° C./min temperature ramp,(B) XRD at 25° C. after reaction with NO₂ at 600° C. and 800° C. forNa_(x)B_(1-x)O_(1.5-x) (x=0.75), (C) Isothermal uptake and release at600° C. for Na_(x)B_(1-x)O_(1.5-x) (x=0.75), and (D)(Li_(0.5)Na_(0.5))_(x)B_(1-x)O_(1.5-x) (x=0.75) under 5° C./mintemperature ramp.

FIG. 8 shows a design of a carbon capture system using molten alkalimetal borates without consideration for other acid gases separation ofsulfurous species at high temperature recovery and purification atambient temperatures, according to certain embodiments.

DETAILED DESCRIPTION

The removal of acid gases (including carbon dioxide and acid gases thatare not carbon dioxide) from industrial streams may find applications inthe energy and chemicals industries, in particular the environmentallyresponsible production of energy from fossil fuels. Acid gases may beenvironmental pollutants, as greenhouse gases or producers of acid rain,and in many cases, these acid gases may be severely harmful to humanhealth. Streams often contain multiple acid gases at high temperatures;however, in conventional systems multiple separate low temperatureprocesses are typically deployed in series to treat each acid gas one ata time. A method by which multiple acid gases can be captured andseparated at high temperatures, without detrimentally impacting theperformance of the system is therefore of keen interest and is describedbelow and elsewhere herein.

In certain embodiments, molten alkali metal borates can be used assorbents to remove acid gas(es) that are not CO₂ (also referred toherein as non-CO₂ acid gas(es)) from streams. Certain embodiments arerelated to the application of molten alkali metal borates in acontinuously circulating system for the removal and separation ofmultiple acid gases at high temperatures. In accordance with certainembodiments, each acid gas interacts differently with the molten alkalimetal borates such that each species can be separated from others atdistinct points in the high temperature system. In certain embodiments,the product streams are upconentrated at high temperatures either byrelease as a gas or physical separation of the solids from therecirculating liquid.

Certain aspects of the present disclosure are directed to the removal ofnon-CO₂ acid gases using a sorbent that include salt in molten form. Insome embodiments, the sorbent may act as a sequestration material forone or more non-CO₂ acid gases. In some embodiments, the removal of thenon-CO₂ acid gas may occur at an elevated temperature (e.g., at or abovethe melting temperature of the salt, such that at least the unreactedmolten salt remains in molten form). The inventors have appreciated andunderstood that certain sorbents described herein may remove a varietyof acid gases, including carbon dioxide. In certain cases, certainsorbents may sequester non-CO₂ acid gases. In addition, in someembodiments, certain sorbents may preferentially sequester non-CO₂ acidgases over carbon dioxide, which may advantageously be useful inseparating carbon dioxide from non-CO₂ acid gases.

While much of the disclosure herein is focused on the treatment ofnon-CO₂ acid gases, it should be understood that the sorbents describedherein may also sequester (in addition to the non-CO₂ acid gas) carbondioxide.

In accordance with certain embodiments, a sorbent may be exposed to anenvironment containing a non-CO₂ acid gas. A non-CO₂ acid gas is anyacid gas that is not carbon dioxide. Non-limiting examples of non-CO₂acid gases include, sulfur monoxide (SO), sulfur dioxide (SO₂), nitrogendioxide (NO₂), hydrogen sulfide (H₂S), sulfur trioxide (SO₃), nitricoxide (NO), nitrous oxide (N₂O), dinitrogen trioxide (N₂O₃), dinitrogentetroxide (N₂O₄), dinitrogen pentoxide (N₂O₅), and/or carbonyl sulfide(COS). Exposure of the sorbent to (and removal of) other acid gases isalso possible.

According to certain embodiments, the sorbent is exposed to the non-CO₂acid gas under conditions favoring sequestration of the non-CO₂ acidgas. For example, in accordance with certain embodiments, a sorbent thatcomprises a salt in molten form can be exposed to the environmentcontaining the non-CO₂ acid gas in a manner facilitating high contactbetween the two, e.g., the sorbent can be flowed (optionally flowedcontinuously) and/or sprayed during exposure of the sorbent to anenvironment containing the non-CO₂ acid gas. The flowing and/or sprayingof the sorbent, during exposure of the sorbent to an environmentcontaining the non-CO₂ acid gas, may advantageously increase the rate ofthe non-CO₂ acid gas capture by the sorbent relative to the rate of thenon-CO₂ acid gas capture by an entirely solid sorbent. For example, thesorbent comprising a salt in molten form may be flowed and/or sprayed inone direction while an environment comprising the non-CO₂ acid gas isflowed in a different direction, e.g., in the opposite direction, in acrosscurrent or countercurrent type operation to maximize heat and/ormass transfer between the sorbent and the environment.

Uptake of the non-CO₂ acid gas by a sorbent in accordance with theinvention can be at any of a variety of desirable levels. Uptake by asorbent comprising a salt in molten form, with the salt including analkali metal cation and a boron oxide anion and/or a dissociated formthereof, may be as much as or greater than 5 mmol of the non-CO₂ acidgas per gram of sorbent within 1 minute of exposure to an environmentcontaining the non-CO₂ acid gas, a significantly faster rate of uptakethan for solid particulate sorbents of similar composition under similarconditions.

In addition, the ability to flow the sorbent comprising a salt in moltenform facilitates, in accordance with certain embodiments, a continuoussequestration process of non-carbon dioxide acid gas(es), in which anon-CO₂ acid gas-loaded sorbent can be flowed from an adsorber vessel toa desorber vessel, and/or an unloaded sorbent can be flowed from thedesorber vessel to the adsorber vessel, for a plurality of cycleswithout halting the process. Continuous operation provides, in someembodiments, advantages including but not limited to a reduced durationof a non-CO₂ acid gas capture process, potentially reduced energy inputrequired in the non-CO₂ acid gas capture process, and the ability torefresh poisoned sorbent with a purge rather than taking a unit offline.As is described elsewhere herein, a non-CO₂ acid gas or a mixture ofnon-CO₂ acid gas may also contain at least some carbon dioxide. Certainof the methods described herein may advantageously be used to separateCO₂ from non-CO₂ acid gases and/or one type of non-CO₂ acid gas fromother types of non-CO₂ acid gases.

Another important advantage associated with the use of a sorbentcomprising a salt in molten form, in accordance with certainembodiments, is the ability to use the sorbent at an elevatedtemperature, e.g., at a temperature greater than or equal to the meltingtemperature of the sorbent, e.g., greater than or equal to 200° C. Thetemperature can be higher as well, e.g., greater than or equal to 250°C., greater than or equal to 300° C., greater than or equal to 350° C.,greater than or equal to 400° C., greater than or equal to 450° C., orgreater than or equal to 500° C., or higher. In some embodiments inwhich the sorbent is used at an elevated temperature, any of a varietyof suitable amounts of the sorbent (e.g., greater than or equal to 1 wt%, greater than or equal to 10 wt %, greater than or equal to 50 wt %,greater than or equal to 75 wt %, greater than or equal to 90 wt %,greater than or equal to 99 wt %, or all of the sorbent) will be at thatelevated temperature (e.g., greater than or equal to 200° C., greaterthan or equal to 250° C., greater than or equal to 300° C., greater thanor equal to 350° C., greater than or equal to 400° C., greater than orequal to 450° C., greater than or equal to 500° C., and/or within any ofthe other temperature ranges mentioned above or elsewhere herein). Asused herein, temperature of operation refers to the temperature of thesorbent itself, which can be essentially equal to or different from thetemperature of the environment to which the sorbent is exposed.

In certain embodiments, the process can optionally take place in apressure swing operation. Generally, in a pressure swing operation incertain embodiments described herein, the sorbent is exposed to anenvironment having a first partial pressure of non-CO₂ acid gas, duringexposure of the sorbent to an environment containing the acid gas, andsubsequently the non-CO₂ acid gas-loaded sorbent is exposed to a secondenvironment having second lower partial pressure of the non-CO₂ acid gas(e.g., 0 bar of the non-CO₂ acid gas), regenerating unloaded sorbent.This pressure swing operation may be repeated for a plurality of cyclesonce the sorbent has been regenerated. The first partial pressure of thenon-CO₂ acid gas may be, in some embodiments, at least 0.000001 bar, atleast 0.0001 bar, at least 0.01 bar, or at least 1 bar. The firstpartial pressure of the non-CO₂ acid gas may be, in some embodiments, atmost 30 bar, at most 20 bar, at most 10 bar, or at most 5 bar.Combinations of the above-referenced ranges are also possible (e.g.,between or equal to 0.000001 bar and 30 bar, between or equal to 0.01bar and 20 bar, between or equal to 0.1 bar and 10 bar, between or equalto 1 bar and 5 bar). Other ranges are also possible. The second partialpressure of the non-CO₂ acid gas may be, in some embodiments, less thanthe first partial pressure of the non-CO₂ acid gas by at least 0.000001bar, at least 0.0001 bar, at least 0.01 bar, or at least 1 bar. Thesecond partial pressure of the non-CO₂ acid gas may be, in someembodiments, less than the first partial pressure of the non-CO₂ acidgas by at most 30 bar, at most 20 bar, at most 10 bar, or at most 5 bar.Combinations of the above-referenced ranges are also possible (e.g.,between or equal to 0.001 bar and 30 bar less, between or equal to 0.01bar and 20 bar less, between or equal to 0.1 bar and 10 bar less,between or equal to 1 bar and 5 bar less). Other ranges are alsopossible.

The process can optionally take place in a temperature swing operation.Generally in a temperature swing operation, in accordance with certainembodiments described herein, the sorbent is exposed to a firsttemperature, during exposure of the sorbent to an environment containingnon-CO₂ acid gas, and subsequently the non-CO₂ acid gas loaded sorbentis exposed to a second higher temperature in a second environmentcontaining less or no non-CO₂ acid gas, regenerating unloaded sorbent.This temperature swing operation may be repeated for a plurality ofcycles once the sorbent has been regenerated. The first temperature maybe greater than or equal to the melting temperature of the sorbent,e.g., greater than or equal to 200° C. The first temperature can behigher as well, e.g., greater than or equal to 250° C., greater than orequal to 300° C., greater than or equal to 350° C., greater than orequal to 400° C., greater than or equal to 450° C., or greater than orequal to 500° C. or higher, and/or less than or equal to 1000° C. Insome embodiments, the second temperature is equal to the firsttemperature. The second temperature may be, in some embodiments, greaterthan the first temperature by at least 10° C., at least 50° C., at least100° C., at least 200° C., at least 300° C., at least 400° C., or atleast 500° C. The second temperature may be, in some embodiments,greater than the first temperature by at most 1000° C., at most 900° C.,at most 800° C., at most 700° C., or at most 600° C. Combinations of theabove-referenced ranges are also possible (e.g., between or equal to 10°C. and 300° C. greater, between or equal to 200° C. and 400° C. greater,between or equal to 400° C. and 1000° C. greater). Other ranges are alsopossible.

It is noted that unless specified otherwise, temperatures and otherconditions described herein are at approximately atmospheric pressure,although deviation from atmospheric pressure can occur while stillmeeting the objectives of the invention. Those of ordinary skill canselect different pressures to achieve the results outlined herein.

Certain embodiments are related to a sorbent material. As used herein,the phrase “sorbent” is used to describe a material that is capable ofremoving non-CO₂ acid gas (optionally along with CO₂) from anenvironment containing non-CO₂ acid gas. In some embodiments, thesorbent may function as a sequestration material.

Certain aspects are related to a sorbent that comprises a salt in moltenform, the composition of which salt can be selected to have a lowmelting temperature relative to other salts such that less energy isrequired to melt the salt. In addition, the composition of the salt canbe selected in order to tune the melting point (e.g., meltingtemperature at 1 atm) of the salt, e.g., to approach or match thetemperature at which the non-CO₂ acid gas, to which the salt is exposed,is emitted from a source of non-CO₂ acid gas.

In certain embodiments, the salt is in molten form. For example, in someembodiments, a solid salt comprising an alkali metal cation and a boronoxide anion (and/or a dissociated form thereof) can be heated above itsmelting temperature which results in the solid transitioning into aliquid state. According to certain embodiments, the salt comprising analkali metal cation and a boron oxide anion (and/or a dissociated formthereof) is a salt having a melting point between or equal to 200° C.and 1000° C. (or between 200° C. and 700° C.) when at atmosphericpressure. Those of ordinary skill in the art would understand that amolten salt is different from a solubilized salt (i.e., a salt that hasbeen dissolved within a solvent).

The salt in molten form can have a number of chemical compositions.According to certain embodiments, the salt in molten form comprises atleast one alkali metal cation and at least one boron oxide anion and/ora dissociated form thereof.

The term “alkali metal” is used herein to refer to the following sixchemical elements of Group 1 of the periodic table: lithium (Li), sodium(Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).

In some embodiments, the at least one alkali metal cation comprisescationic lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and/orcesium (Cs). In some embodiments, the at least one alkali metal cationcomprises cationic lithium (Li), sodium (Na), and/or potassium (K).

In some embodiments, the salt in molten form comprises at least oneother metal cation. In some embodiments, the at least one other metalcation comprises an alkali metal cation, an alkaline earth metal cation,or a transition metal cation. In some embodiments, the salt in moltenform comprises at least two alkali metal cations (e.g., 3 alkali metalcations). In certain embodiments, the salt in molten form comprisescationic lithium and cationic sodium.

A salt in molten form comprising cationic lithium and cationic sodiummay in some embodiments provide advantages in a temperature swingoperation, e.g., relative to an analogous salt in molten form comprisingcationic sodium or an analogous salt in molten form comprising cationiclithium, cationic sodium, and cationic potassium. One advantage of asalt in molten form comprising cationic lithium and cationic sodium maybe a higher uptake capacity of non-CO₂ acid gas, in a temperature rangeof between or equal to 500° C. and 700° C., than an analogous salt inmolten form comprising cationic sodium or an analogous salt in moltenform comprising cationic lithium, cationic sodium, and cationicpotassium. Another advantage of a salt in molten form comprisingcationic lithium and cationic sodium may be that a lesser temperaturedifference can be employed in a temperature swing operation for the sameregeneration efficiency of the capture and release of non-CO₂ acid gas(e.g., a temperature difference of between or equal to 0.25 and 0.5times the temperature difference employed for analogous salts) relativeto an analogous salt in molten form comprising cationic sodium or ananalogous salt in molten form comprising cationic lithium, cationicsodium, and cationic potassium.

The term “alkaline earth metal” is used herein to refer to the sixchemical elements in Group 2 of the periodic table: beryllium (Be),magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium(Ra).

The “transition metal” elements are scandium (Sc), yttrium (Y),lanthanum (La), actinium (Ac), titanium (Ti), zirconium (Zr), hafnium(Hf), rutherfordium (Rf), vanadium (V), niobium (Nb), tantalum (Ta),dubnium (Db), chromium (Cr), molybdenum (Mo), tungsten (W), seaborgium(Sg), manganese (Mn), technetium (Tc), rhenium (Re), bohrium (Bh), iron(Fe), ruthenium (Ru), osmium (Os), hassium (Hs), cobalt (Co), rhodium(Rh), iridium (Ir), meitnerium (Mt), nickel (Ni), palladium (Pd),platinum (Pt), darmstadtium (Ds), copper (Cu), silver (Ag), gold (Au),roentgenium (Rg), zinc (Zn), cadmium (Cd), mercury (Hg), and copernicium(Cn).

In certain embodiments, it may be advantageous for the salt in moltenform to comprise an alkali metal cation and one other metal cation at acomposition at or near a eutectic composition, such that the meltingtemperature of the salt is lower than the melting temperature of a saltwith a different composition of the alkali metal cation and the oneother metal cation, reducing the energy required to attain the salt inmolten form for an operation for the sequestration of non-CO₂ acidgas(es).

Certain of the sorbents described herein have relatively low meltingtemperatures and may promote sequestration (e.g., absorption) of non-CO₂acid gas at relatively low temperatures. For example, components thatare capable of forming eutectic compositions with each other havereduced melting points at the eutectic composition and at compositionssurrounding the eutectic composition in comparison to compositions inwhich the components are present in other relative amounts. As anotherexample, compositions comprising alkali metal cations and/or alkalineearth metal cations have relatively low melting points in comparison tocompositions comprising other metal cations. The ability to absorbnon-CO₂ acid gas at relatively low temperatures can be advantageous asit may, according to certain although not necessarily all embodiments,reduce the amount of energy required to absorb acid gases.

In some embodiments the sorbent comprises at least two components (e.g.,metal cations, alkali metal cation(s)) that are capable of forming aeutectic composition with each other. As would be understood by one ofordinary skill in the art, a “eutectic composition” is a compositionthat melts at a temperature lower than the melting points of thecomposition's constituents. In some eutectic compositions, the liquidphase is in equilibrium with both a first solid phase and a second solidphase different from the first solid phase at the eutectic temperature.A eutectic composition that is cooled from a temperature above theeutectic temperature to a temperature below the eutectic temperatureunder equilibrium cooling conditions undergoes, in certain cases,solidification at the eutectic temperature to form a first solid phaseand a second solid phase simultaneously from a liquid. As would also beunderstood by one of ordinary skill in the art, two components that arecapable of forming a eutectic composition with each other are, incertain cases, also able to form non-eutectic compositions with eachother. Non-eutectic compositions often undergo solidification over arange of temperatures because liquid phases may be in equilibrium withsolid phases over a range of temperatures.

The term “boron oxide anion” is used herein to refer to a negativelycharged ion comprising at least one boron and at least one oxygen. Theboron oxide anion in the salt in molten form can be intact (e.g., inanionic B_(w)O_(z) form, e.g., (BO₃ ³⁻)) and/or the boron and oxygen canbe dissociated from one another (e.g., into boron cation(s) and oxygenanion(s), e.g., as B³⁺ and O²⁻).

According to some embodiments, the at least one boron oxide anioncomprises anionic B_(w)O_(z) and/or a dissociated form thereof. In someembodiments, w is greater than 0 and less than or equal to 4. In certainembodiments, w is between or equal to 1 and 4. In some embodiments, z isgreater than 0 and less than or equal to 9. In certain embodiments, z isbetween or equal to 1 and 9. In some embodiments, the at least one boronoxide anion comprises anionic BO₃, BO₄, or B₂O₅ and/or a dissociatedform thereof. In certain embodiments, it may be advantageous to have asalt in molten form comprise anionic BO₃ and/or a dissociated formthereof. A potential advantage of anionic BO₃ and/or a dissociated formthereof may include a greater acid gas uptake capacity of the salt inmolten form during exposure to an environment containing acid gas,relative to a salt having the same alkali metal cation (and any othercations) and anionic B₂O₅ and/or a dissociated form thereof. Anotherpotential advantage of anionic BO₃ and/or a dissociated form thereof mayinclude a greater acid gas desorption of the salt in molten form duringexposure to desorption conditions, relative to a salt having the samealkali metal cation (and any other cations) and anionic BO₄ and/or adissociated form thereof.

In some embodiments, the boron oxide anion comprises B_(w)O_(z) and/or adissociated form thereof, wherein w is greater than 0 and less than orequal to 4 and z is greater than 0 and less than or equal to 9.

In some embodiments, the fractional stoichiometry of a salt describedherein can be expressed as M_(x)B_(1-x)O_(y), wherein x is a mixingratio and is between zero and 1. In some embodiments, the fractionalstoichiometry is that of the salt in solid form, e.g., before melting.In some embodiments, the fractional stoichiometry is that of the salt inmolten form, e.g., after melting. In certain embodiments, y=1.5−x. “M”in this formula refers to the metal cation(s) (e.g., an alkali metalcation, a combination of an alkali metal cation and at least one othermetal cation) in a sorbent described herein. For example, in someembodiments, the fractional stoichiometry of a salt described herein canbe expressed as A_(x)B_(1-x)O_(y), where 0<x<1 and A is an alkali metal(e.g., Li, Na, K). In certain such embodiments, y=1.5−x.

As used herein, the term “mixing ratio” of an alkali metal cation orcombination of metal cations in a sorbent refers to the ratio of molesof metal cation(s) in a sorbent to the total of moles of metal cation(s)plus moles of boron in the sorbent. For example, the mixing ratio ofsodium in Na₃BO₃ is 3/(3+1)=0.75; the mixing ratio of alkali metals in(Li_(0.5)Na_(0.5))₃BO₃ is (0.5*3+0.5*3)/(3+1)=0.75. In some embodiments,the mixing ratio is at least 0.5, at least 0.6, or at least 0.667. Insome embodiments, the mixing ratio is at most 0.9, at most 0.835, atmost 0.8, at most 0.75, or at most 0.7. Combinations of theabove-referenced ranges are also possible (e.g., between or equal to 0.5and 0.9, between or equal to 0.6 and 0.8, between or equal to 0.7 and0.8). Other ranges are also possible. Without wishing to be bound bytheory, there may be a mixing ratio (for a certain alkali metal cationor combination of metal cations) below which the acid gas uptakecapacity of the sorbent is less than desirable. Without wishing to bebound by theory, there may be a mixing ratio (for a certain alkali metalcation or combination of metal cations) above which the regenerationefficiency of the sorbent is less than desirable. In some embodiments,the alkali metal comprises lithium (Li), sodium (Na), potassium (K),and/or a mixture of these. In some embodiments, the alkali metalcomprises Li and Na in equal amounts.

Non-limiting examples of the salt in molten form include but are notlimited to Na₃BO₃ (which could also be written as, e.g.,Na_(0.75)B_(0.5)O_(0.75)), Na₅BO₄ (which could also be written as, e.g.,Na_(0.83)B_(0.17)O_(0.67)), Na₄B₂O₅ (which could also be written as,e.g., Na₂BO_(2.5)), K₃BO₃ (which could also be written as, e.g.,K_(0.75)B_(0.25)O_(0.75)), (Li_(0.5)Na_(0.5))₃BO₃, and/or(Li_(0.33)Na_(0.33)K_(0.33))₃BO₃, or a combination thereof, in moltenform.

In some embodiments, the salt of the sorbent that is in molten form maybe accompanied by portions of that salt that are not molten. That is tosay, complete melting of all of the salt type(s) that are present inmolten form is not required in all embodiments. In some embodiments, atleast 10 wt %, at least 20 wt %, at least 30 wt %, at least 40 wt %, atleast 50 wt %, at least 60 wt %, at least 70 wt %, at least 80 wt %, atleast 90 wt %, or more of the salt present within the sorbent is molten.In some embodiments, less than 100 wt %, less than 99 wt %, less than 90wt %, or less of the salt that is present within the sorbent is molten.Combinations of the above-referenced ranges are also possible (e.g., atleast 10 wt % and less than 100 wt %). Other ranges are also possible.

In some embodiments, the sorbent comprises at least one salt comprisingat least one alkali metal cation and at least one boron oxide anionand/or a dissociated form thereof (e.g., including, but not limited to,Na₃BO₃, Na₅BO₄, Na₄B₂O₅, K₃BO₃, (Li_(0.5)Na_(0.5))₃BO₃, and/or(Li_(0.33)Na_(0.33)K_(0.33))₃BO₃) for which at least 10 wt %, at least20 wt %, at least 30 wt %, at least 40 wt %, at least 50 wt %, at least60 wt %, at least 70 wt %, at least 80 wt %, at least 90 wt %, or moreof that salt is molten. In some embodiments, the sorbent comprises atleast one salt comprising at least one alkali metal cation and at leastone boron oxide anion and/or a dissociated form thereof (e.g.,including, but not limited to, Na₃BO₃, Na₅BO₄, Na₄B₂O₅, K₃BO₃,(Li_(0.5)Na_(0.5))₃BO₃, and/or (Li_(0.33)Na_(0.33)K_(0.33))₃BO₃) forwhich less than 100 wt %, less than 99 wt %, less than 90 wt %, or lessof that salt is molten. Combinations of the above-referenced ranges arealso possible. Other ranges are also possible.

In some embodiments, in the sorbent, the total amount of all salts thatcomprise at least one alkali metal cation and at least one boron oxideanion and/or a dissociated form thereof and that is in molten form is atleast 10 wt %, at least 20 wt %, at least 30 wt %, at least 40 wt %, atleast 50 wt %, at least 60 wt %, at least 70 wt %, at least 80 wt %, atleast 90 wt %, or more. As a non-limiting exemplary illustration, insome embodiments, the sorbent can be a combination of 50 grams ofNa₃BO₃, 50 grams of Na₅BO₄, and 50 grams of Na₄B₂O₅, and in some suchembodiments, at least 15 grams (i.e., 10 wt % of 150 total grams) of thetotal amount of Na₃BO₃, Na₅BO₄, and Na₄B₂O₅ is molten. In certainembodiments, in the sorbent, the total amount of all salts that compriseat least one alkali metal cation and at least one boron oxide anionand/or a dissociated form thereof and that is in molten form is lessthan 100 wt %, less than 99 wt %, less than 90 wt %, less than 50%, lessthan 40%, less than 30%, less than 20%, or less. Combinations of theabove-referenced ranges are also possible (e.g., at least 10 wt % andless than 100 wt %). Other ranges are also possible.

In some embodiments, a sorbent further comprises an additive. Examplesof types of additives that may be included in a sorbent include but arenot limited to corrosion inhibitors, viscosity modifiers, wettingagents, high-temperature surfactants, and scale inhibitors. In someembodiments, the sorbent comprises a plurality of additives (e.g., two,three, four, or more).

In some embodiments, during exposure to an environment comprising anon-CO₂ acid gas, at least a portion of the salt in molten formchemically reacts with at least some of the non-CO₂ acid gas and formsone or more products (e.g., comprising a carbonate, comprising nitrate,comprising nitrite, comprising sulfate, comprising sulfite) within thesorbent. These one or more products (e.g., carbonate products, nitrateproducts, nitrate products, sulfate products, sulfite products) may bein solid form or in liquid form, depending, e.g., on the temperatureand/or composition of the salt (e.g., alkali metal borate salt).

In some embodiments, during exposure to an environment comprisingnon-CO₂ acid gas, at least a portion of the salt in molten formchemically reacts with at least some of the non-CO₂ acid gas(es) andforms solid particles (e.g., comprising a carbonate, a sulfate, asulfite, a nitrate, a nitrite) within the sorbent, increasing theviscosity of the sorbent. These solid particles loaded with non-CO₂ acidgas(es) may be flowed within remaining salt in molten form using aslurry pump to a desorber to be regenerated (e.g., regeneration of saltin molten form from the solid particulates), or alternatively thesesolid particles may be regenerated within the same vessel in which thesolid particles were formed.

In some embodiments, a relatively large percentage of the sorbent ismade up of a salt in molten form. For example, in some embodiments, atleast 10 weight percent (wt %), at least 20 wt %, at least 30 wt %, atleast 40 wt %, at least 50 wt %, at least 60 wt %, at least 70 wt %, atleast 80 wt %, at least 90 wt %, or more of the sorbent is made up of asalt in molten form. In some embodiments, at most 100 wt %, at most 99wt %, or at most 90 wt % of the sorbent is made up of a salt in moltenform. Combinations of the above-referenced ranges are also possible(e.g., between or equal to 10 wt % and 100 wt %, between or equal to 20wt % and 99 wt %, between or equal to 50 wt % and 90 wt %). Other rangesare also possible. In some embodiments, all of the sorbent is molten. Inother embodiments, only a portion of the sorbent is molten.

In some embodiments, a relatively large percentage of the sorbent ischemically converted to non-CO₂ acid gas-loaded solid particles duringsequestration (e.g., absorption). For example, in some embodiments, atleast 1 wt %, at least 10 wt %, or at least 20 wt % of the sorbent ismade up of non-CO₂ acid gas-loaded solid particles. In some embodiments,at most 90 wt %, at most 80 wt %, or at most 50 wt % of the sorbent ismade up of non-CO₂ acid gas-loaded solid particles. Combinations of theabove-referenced ranges are also possible (e.g., between or equal to 1wt % and 90 wt %, between or equal to 10 wt % and 80 wt %, between orequal to 10 wt % and 50 wt %, between or equal to 20 wt % and 50 wt %).Other ranges are also possible.

In some embodiments, the sorbent also comprises a hydroxide of an alkalimetal. For example, in some embodiments, the sorbent comprises NaOH,KOH, and/or LiGH. According to certain embodiments, a hydroxide of analkali metal can be formed as a by-product of a reaction between thesorbent and a non-CO₂ acid gas.

According to certain embodiments, the sorbent is capable of interactingwith a non-CO₂ acid gas such that a relatively large amount of thenon-CO₂ acid gas is sequestered. In certain embodiments, the sorbent iscapable of interaction with mixtures of non-CO₂ acid gases. In someembodiments, the sorbent may preferentially sequester non-CO₂ acidgas(es) over CO₂, and thus may facilitate the separation of non-CO₂ acidgases from CO₂. Interaction between the sorbent and acid gases (e.g.,non-CO₂ acid gases) can involve a chemical reaction, adsorption, and/ordiffusion. In some embodiments, a plurality of non-CO₂ acid gasesinteracts with the sorbent such that at least a portion of the pluralityof non-CO₂ acid gases are removed from the environment.

For example, in certain embodiments, the sorbent is capable ofinteracting with a non-CO₂ acid gas such that at least 0.01 mmol of thenon-CO₂ acid gas is sequestered (e.g., from an environment, e.g., froman atmosphere, from a stream) per gram of the sorbent. In someembodiments, the sorbent is capable of interacting with a non-CO₂ acidgas such that at least 0.1 mmol, at least 0.5 mmol, at least 2.0 mmol,or at least 10.0 mmol of the non-CO₂ acid gas is sequestered (e.g., froman environment, e.g., from an atmosphere, from a stream) per gram of thesorbent. In certain embodiments, the sorbent is capable of interactingwith the non-CO₂ acid gas such that at most 20.0 mmol, at most 18.0mmol, at most 16.0 mmol, at most 14.0 mmol, or at most 12.0 mmol of theacid gas is sequestered (e.g., from an environment, e.g., from anatmosphere, from a stream) per gram of the sorbent. Combinations of theabove-referenced ranges are also possible (e.g., between or equal to 0.1mmol per gram and 20.0 mmol per gram, between or equal to 0.5 mmol pergram and 16.0 mmol per gram, between or equal to 2.0 mmol per gram and12.0 mmol per gram).

According to certain embodiments, the sorbent is capable of interactingwith the non-CO₂ acid gas such that a relatively large amount of thenon-CO₂ acid gas is sequestered even when the non-CO₂ acid gasconcentration in the environment (e.g., in an atmosphere, in a stream)is relatively low. For example, in some embodiments, the sorbent iscapable of sequestering at least 0.01 mmol, at least 0.1 mmol, at least0.5 mmol, at least 2.0 mmol, at least 10.0 mmol and/or at most 20.0mmol, at most 18.0 mmol, at most 16.0 mmol, at most 14.0 mmol, or atmost 12.0 mmol of the non-CO₂ acid gas per gram of the sorbent when thesorbent is exposed to a steady state environment containing as little as50 mol %, as little as 25 mol %, as little as 10 mol %, or as little as1 mol % of the non-CO₂ acid gas (e.g., with the balance of theenvironment being argon).

According to certain embodiments, the sorbent is capable of interactingwith a non-CO₂ acid gas such that a relatively large amount of thenon-CO₂ acid gas is sequestered even at relatively low temperatures. Forexample, in some embodiments, the sorbent is capable of sequestering atleast 0.01 mmol, at least 0.1 mmol, at least 0.5 mmol, at least 2.0mmol, at least 10.0 mmol and/or at most 20.0 mmol, at most 18.0 mmol, atmost 16.0 mmol, at most 14.0 mmol, or at most 12.0 mmol of the non-CO₂acid gas per gram of the sorbent when the sorbent is at a temperature of1000° C. or less, at a temperature of 850° C. or less, at a temperatureof 600° C. or less, at a temperature of 550° C. or less, or at atemperature of 520° C. or less (and/or, at a temperature of at least200° C., at least 300° C., at least 400° C., at least 450° C., or atleast 500° C.). Combinations of the above-referenced ranges are alsopossible (e.g., between or equal to 200° C. and 1000° C., between orequal to 200° C. and 600° C., between or equal to 400° C. and 550° C.).Other ranges are also possible.

According to certain embodiments, the salt of the sorbent has a meltingtemperature at 1 atm within a range high enough to provide a high rateof sequestration of the non-CO₂ acid gas(es), but not so high as to makethe sequestration of the non-CO₂ acid gas(es) an overly energy-intensiveprocess. In some embodiments, the salt of the sorbent has a meltingtemperature at 1 atm of at least 200° C., at least 300° C., at least400° C., at least 450° C., or at least 500° C. In some embodiments, thesalt of the sorbent has a melting temperature at 1 atm of at most 1000°C., at most 850° C., at most 600° C., at most 550° C., or at most 520°C.

Combinations of the above-referenced ranges are also possible (e.g.,between or equal to 200° C. and 1000° C., between or equal to 200° C.and 600° C., between or equal to 400° C. and 550° C.). Other ranges arealso possible.

According to certain embodiments, the sorbent is capable of interactingwith a non-CO₂ acid gas such that a relatively large amount of thenon-CO₂ acid gas is sequestered over a relatively short period of time.For example, in some embodiments, the sorbent is capable of sequesteringat least 0.01 mmol, at least 0.1 mmol, at least 0.5 mmol, at least 2.0mmol, at least 10.0 mmol and/or at most 20.0 mmol, at most 18.0 mmol, atmost 16.0 mmol, at most 14.0 mmol, or at most 12.0 mmol of the non-CO₂acid gas per gram of the sorbent when the sorbent is exposed to anenvironment containing the non-CO₂ acid gas for a period of 24 hours orless, 12 hours or less, 8 hours or less, 4 hours or less, 1 hour orless, 30 minutes or less, 10 minutes or less, or 2 minutes or less(and/or, at least 10 seconds, at least 20 seconds, at least 30 seconds,or at least 1 minute). Combinations of the above-referenced ranges arealso possible (e.g., between or equal to 10 seconds and 24 hours,between or equal to 20 seconds and 12 hours, between or equal to 30seconds and 8 hours, between or equal to 1 minute and 4 hours, betweenor equal to 1 minute and 10 minutes, between or equal to 1 minute and 2minutes). Other ranges are also possible.

The amount of non-CO₂ acid gas sequestered by a sorbent can bedetermined, for example, using thermogravimetric analysis.

In addition to sorbents, methods of capturing non-CO₂ acid gas usingsorbents are also described. For example, certain of the sorbentsdescribed herein can be used to remove non-CO₂ acid gas from a chemicalprocess stream (e.g., the exhaust stream of a combustion system) and/orfrom an environment containing non-CO₂ acid gas (e.g., an environmentwithin a reactor or other unit operation).

In some embodiments, a method comprises melting a solid sorbentcomprising a salt described herein (e.g., an alkali metal borate), andusing the molten sorbent to sequester a non-CO₂ acid gas. In someembodiments, the salt (e.g., alkali metal borate) in molten formcomprises an alkali metal cation, a boron oxide anion, a boron cation,and/or an oxygen anion. In certain embodiments, all of these species arepresent in the salt in molten form. In some embodiments, the salt (e.g.,alkali metal borate) in molten form comprises an alkali metal cation, aboron cation, and an oxygen anion.

Certain aspects are related to methods of sequestering a non-CO₂ acidgas using a sorbent described herein. Certain aspects are directed to amethod comprising exposing a sorbent described herein to an environmentcontaining the non-CO₂ acid gas such that at least some of the non-CO₂acid gas interacts with the sorbent and at least a portion of thenon-CO₂ acid gas is sequestered from the environment. In some suchembodiments, a relatively large percentage of the non-CO₂ acid gas(e.g., at least 25 wt %, at least 50 wt %, at least 75 wt %, at least 90wt %, at least 95 wt %, at least 99 wt %, at least 99.9 wt %, or more ofthe non-CO₂ acid gas) is removed from the environment. In certainembodiments, essentially all of the non-CO₂ acid gas is removed from theenvironment.

In certain embodiments, a method comprises exposing a sorbent at atemperature of at least 200° C. to an environment containing non-CO₂acid gas such that at least some of the non-CO₂ acid gas interacts withthe sorbent and is sequestered from the environment.

The sorbent can be exposed to an environment containing a non-CO₂ acidgas in a number of ways. For example, in some embodiments, the sorbentcan be added to an environment (e.g., an atmosphere, a stream)containing the non-CO₂ acid gas. According to certain embodiments, theenvironment containing the non-CO₂ acid gas can be transported into(e.g., flowed through) a container holding the sorbent. In certainembodiments, the sorbent comprising a salt in molten form can be flowedor sprayed through a container in which the environment resides and/oris flowed in the same and/or opposite direction to the flow direction orspray direction of the sorbent. Combinations of these methods are alsopossible. The non-CO₂ acid gas to which the sorbent is exposed isgenerally in fluidic form (e.g., in the form of a gas and/or asupercritical fluid). In certain embodiments, at least a portion of thenon-CO₂ acid gas to which the sorbent is exposed is in the form of asubcritical gas.

The environment containing a non-CO₂ acid gas to which the sorbent isexposed can be, for example, contained within a chemical processing unitoperation. Non-limiting examples of such unit operations includereactors (e.g., packed bed reactors, fluidized bed reactors, fallingfilm columns, bubble columns), separators (e.g., particulate filters,such as diesel particulate filters), and mixers. According to certainembodiments, the environment containing the non-CO₂ acid gas iscontained within a falling film column. According to certainembodiments, the environment containing the non-CO₂ acid gas is part ofand/or derived from the output of a combustion process.

In certain embodiments, a method comprises exposing the sorbent to astream containing a non-CO₂ acid gas (optionally also containing CO₂).FIG. 1 is, in accordance with certain embodiments, a schematic diagramof a sorbent being exposed to an environment containing non-CO₂ acidgas. As shown in FIG. 1, method 100 a may comprise exposing sorbent 102to stream 104 a containing non-CO₂ acid gas. The stream to which thesorbent is exposed can be, for example, part of and/or derived from astream of a chemical process containing non-CO₂ acid gas. For example,in some embodiments, the stream to which the sorbent is exposed can bepart of and/or derived from an output (e.g., an exhaust stream) of acombustion process. FIG. 2 is, in accordance with certain embodiments, aschematic diagram of a sorbent being exposed to an environmentcontaining a non-CO₂ acid gas that is part of and/or derived from theoutput of a combustion process. As shown in FIG. 2, method 100 b maycomprise exposing sorbent 102 to stream 104 b containing the non-CO₂acid gas that is part of and/or derived from the output of combustionprocess 108. In some embodiments, at least a portion of an output streamof a combustion process is directly transported through the sorbent. Forexample, as shown in FIG. 2, at least a portion of stream 104 b ofcombustion process 108 is directly transported through sorbent 102.

The stream to which the sorbent is exposed can be, for example,transported through a chemical processing unit operation. Non-limitingexamples of such unit operations include reactors (e.g., packed bedreactors, fluidized bed reactors, falling film columns, bubble columns),separators (e.g., particulate filters, such as diesel particulatefilters), and mixers.

For example, referring back to FIG. 1, in some embodiments, sorbent 102is located within optional reactor 110. According to certainembodiments, the stream to which the sorbent is exposed is transportedthrough a falling film column.

The sorbents described herein can be used to remove non-CO₂ acid gasgenerated by a variety of systems. For example, in some embodiments, thesorbent is used to remove non-CO₂ acid gas from an exhaust stream from aboiler (e.g., in a power plant), from an exhaust stream from anintegrated gasification combined cycle (IGCC) power plant, from anexhaust stream from an internal combustion engine (e.g., from a motorvehicle), from an exhaust stream from a pyro-processing furnace (e.g.,as used in the cement industry), and/or from a stream from a hydrogengeneration process (e.g., by sorption enhanced steam reforming (SESR)).

The concentration of non-CO₂ acid gas in the fluid to which the sorbentis exposed can be within a variety of ranges. In some embodiments, theenvironment (e.g., an atmosphere, a stream) to which the sorbent isexposed contains non-CO₂ acid gas in an amount of at least 1 ppm. Incertain embodiments, the environment (e.g., an atmosphere, a stream) towhich the sorbent is exposed contains non-CO₂ acid gas in an amount ofat least 10 ppm, at least 1000 ppm, at least 0.01 mol %, at least 0.1mol %, at least 1 mol %, at least 10 mol %, at least 50 mol %, or atleast 99 mol %. The sorbent can be exposed, in some embodiments, toessentially pure non-CO₂ acid gas. In some embodiments, a methodinvolves exposing the sorbent to an environment that contains non-CO₂acid gas in an amount of at least 1 ppm.

Certain embodiments comprise exposing the sorbent to an environment(e.g., an atmosphere, a stream) comprising non-CO₂ acid gas such that atleast a portion of the non-CO₂ acid gas interacts with the sorbent andat least a portion of the non-CO₂ acid gas is sequestered from theenvironment (e.g., from the atmosphere, from the stream). For example,as shown in FIG. 1, at least a portion of the non-CO₂ acid gas in stream104 a interacts with sorbent 102 and is sequestered from stream 104 a,thereby being absent from stream 106 a. In certain embodiments, stream106 a may contain less non-CO₂ acid gas than stream 104 a after at leasta portion of the non-CO₂ acid gas in stream 104 a interacts with sorbent102 and is sequestered from stream 104 a. The interaction between thenon-CO₂ acid gas that is sequestered and the sorbent can take a varietyof forms. For example, in certain embodiments, the non-CO₂ acid gas isabsorbed into the sorbent. In some embodiments, the non-CO₂ acid gas isadsorbed onto the sorbent. In some embodiments, the non-CO₂ acid gaschemically reacts with the sorbent. In some embodiments, the non-CO₂acid gas diffuses into the sorbent. Combinations of two or more of thesemechanisms (i.e., absorption, adsorption, chemical reaction, and/ordiffusion) are also possible. In some embodiments, sequestration of theCO₂ does not produce a solid precipitant.

In some embodiments, captured non-CO₂ acid gas forms a solid suspendedin the liquid sorbent and is upconcentrated by physical separation. Insome embodiments, the physical separation uses a cross-flow filter. Insome embodiments, the cross-flow filter is operated at a temperature ofat least 200° C. (or at least 400° C., at least 600° C., or at least800° C.). In some embodiments, the cross-flow filter is operated at atemperature of no greater than 1000° C. In some embodiments, theseparation comprises centrifugation. In certain embodiments, theseparation comprises crystallization. In some embodiments, theseparation comprises sedimentation. Combinations of these are alsopossible.

According to certain embodiments, a relatively large amount of a non-CO₂acid gas is sequestered by the sorbent (e.g., from an atmosphere, from astream) during the exposure of the sorbent to the non-CO₂ acid gas. Forexample, in certain embodiments, at least 0.01 mmol of the non-CO₂ acidgas is sequestered (e.g., from an environment, e.g., from an atmosphere,from a stream) per gram of the sorbent. In some embodiments, at least0.1 mmol, at least 0.5 mmol, at least 2.0 mmol, or at least 10.0 mmol ofthe non-CO₂ acid gas is sequestered (e.g., from an environment, e.g.,from an atmosphere, from a stream) per gram of the sorbent. In certainembodiments, at most 20.0 mmol, at most 18.0 mmol, at most 16.0 mmol, atmost 14.0 mmol, or at most 12.0 mmol of the non-CO₂ acid gas issequestered (e.g., from an environment, e.g., from an atmosphere, from astream) per gram of the sorbent. Combinations of the above-referencedranges are also possible (e.g., between or equal to 0.01 mmol per gramand 20.0 mmol per gram, between or equal to 0.1 mmol per gram and 18.0mmol per gram, between or equal to 2.0 mmol per gram and 12.0 mmol pergram). Other ranges are also possible. In some embodiments, including insome methods described herein, between or equal to 0.01 mmol and 20.0mmol of the non-CO₂ acid gas is sequestered from the environment pergram of the sorbent.

According to certain embodiments, at least a portion of the non-CO₂ acidgas interacts with the sorbent and is sequestered from an environmentcontaining the non-CO₂ acid gas, such as from an atmosphere or a streamas noted elsewhere herein, over a period of at least 10 seconds, atleast 20 seconds, at least 30 seconds, or at least 1 minute. Accordingto certain embodiments, at least a portion of the non-CO₂ acid gasinteracts with the sorbent and is sequestered from an environmentcontaining the non-CO₂ acid gas, such as from an atmosphere or a streamas noted elsewhere herein, over a period of 24 hours or less, 12 hoursor less, 8 hours or less, 4 hours or less, 1 hour or less, 30 minutes orless, 10 minutes or less, or 2 minutes or less. Combinations of theabove-referenced ranges are also possible (e.g., between or equal to 10seconds and 24 hours, between or equal to 20 seconds and 12 hours,between or equal to 30 seconds and 8 hours, between or equal to 1 minuteand 4 hours, between or equal to 1 minute and 10 minutes, between orequal to 1 minute and 2 minutes). Other ranges are also possible.

In certain embodiments, at least 0.01 mmol of the non-CO₂ acid gas issequestered (e.g., from the atmosphere, from the stream) per gram of thesorbent per 24 hours. In some embodiments, at least 0.1 mmol, at least0.5 mmol, at least 2.0 mmol, or at least 10.0 mmol of the non-CO₂ acidgas is sequestered from the stream per gram of the sorbent per 24 hours.According to some embodiments, at most 20.0 mmol, at most 18.0 mmol, atmost 16.0 mmol, at most 14.0 mmol, or at most 12.0 mmol of the non-CO₂acid gas is sequestered from the stream per gram of the sorbent per 24hours. Combinations of the above-referenced ranges are also possible(e.g., between or equal to 0.01 mmol per gram and 20.0 mmol per gram,between or equal to 0.1 mmol per gram and 18.0 mmol per gram, between orequal to 2.0 mmol per gram and 12.0 mmol per gram). Other ranges arealso possible.

In certain embodiments, the temperature of the sorbent is less than orequal to 1000° C. during at least a portion of the sequestration of thenon-CO₂ acid gas. In certain embodiments, the sorbent is at atemperature greater than the melting temperature of the salt during atleast a portion of the sequestration of the non-CO₂ acid gas, such thatthe salt is in molten form. In certain embodiments, the temperature ofthe sorbent is at most 1000° C., at most 850° C., at most 600° C., atmost 550° C., or at most 520° C. during at least a portion of thesequestration of the non-CO₂ acid gas. In some embodiments, thetemperature of the sorbent is at least 200° C., at least 300° C., atleast 400° C., at least 450° C., or at least 500° C. during at least aportion of the sequestration of the non-CO₂ acid gas. Combinations ofthe above-referenced ranges are also possible (e.g., between or equal to200° C. and 1000° C., between or equal to 200° C. and 600° C., betweenor equal to 400° C. and 550° C.). Other ranges are also possible.

In certain embodiments, the temperature of the environment containingnon-CO₂ acid gas is less than or equal to 1000° C. during at least aportion of the sequestration of the non-CO₂ acid gas. In certainembodiments, the temperature of the environment containing non-CO₂ acidgas is at most 1000° C., at most 850° C., at most 600° C., at most 550°C., or at most 520° C. during at least a portion of the sequestration ofthe non-CO₂ acid gas. In some embodiments, the temperature of theenvironment containing non-CO₂ acid gas is at least 200° C., at least300° C., at least 400° C., at least 450° C., or at least 500° C. duringat least a portion of the sequestration of the non-CO₂ acid gas.Combinations of the above-referenced ranges are also possible (e.g.,between or equal to 200° C. and 1000° C., between or equal to 200° C.and 600° C., between or equal to 400° C. and 550° C.). Other ranges arealso possible.

In some embodiments, a relatively large weight percentage of the sorbentsequesters non-CO₂ acid gas during sequestration. For example, in someembodiments, at least 0.01 wt %, at least 10 wt %, or at least 20 wt %of the sorbent sequesters non-CO₂ acid gas during sequestration. In someembodiments, at most 100%, at most 90 wt %, at most 80 wt %, or at most50 wt % of the sorbent sequesters non-CO₂ acid gas during sequestration.Combinations of the above-referenced ranges are also possible (e.g.,between or equal to 0.01 wt % and 100 wt %, between or equal to 10 wt %and 90 wt %, between or equal to 10 wt % and 80 wt %, between or equalto 20 wt % and 50 wt %). Other ranges are also possible.

In some embodiments, a method further comprises regenerating the sorbentby removing, from the sorbent, at least 50 mol % of the non-CO₂ acid gassequestered by the sorbent. In some embodiments, at least 60 mol %, atleast 70 mol %, at least 80 mol %, at least 90 mol %, at least 95 mol %,at least 99 mol %, or at least 99.9 mol % of the non-CO₂ acid gassequestered by the sorbent is removed from the sorbent. In someembodiments, the sorbent remains in a liquid state throughout thesequestration and regeneration process. For example, in someembodiments, the salt remains in a liquid state throughout thesequestration and regeneration process.

In certain embodiments, a method comprises performing at least onesequestration/regeneration cycle (e.g., at least one temperature swingcycle, at least one pressure swing cycle). Eachsequestration/regeneration cycle is made up of a sequestration step (inwhich non-CO₂ acid gas is sequestered by the sorbent) followed by aregeneration step (in which non-CO₂ acid gas is released by thesorbent). According to certain embodiments, the sorbent can be subjectto a relatively large number of sequestration/regeneration cycles whilemaintaining the ability to sequester and release relatively largeamounts of non-CO₂ acid gas.

The sorbent can be exposed to any of the environments (e.g.,atmospheres, streams) described above or elsewhere herein during one ormore (or all) of the sequestration steps of the one or moresequestration/regeneration cycles. One or more (or all) of theregeneration steps of the sequestration/regeneration cycles can beperformed using a variety of suitable second environments (e.g., fluids,atmospheres, streams). In some embodiments, regeneration of the sorbentcan be performed by flowing an inert gas (e.g., argon, N₂) over thesorbent. Non-limiting examples of suitable environment components thatcan be used during the regeneration step include a flow of 100 mol % N₂,or a flow of air.

In some embodiments, the gas space in a desorber vessel (furtherdescribed elsewhere herein) comprises a non-CO₂ acid gas, e.g., greaterthan or equal to 0.0001 volume % of the gas space in the desorber vesselis made of non-CO₂ acid gas(es). As would be understood by a person ofordinary skill in the art, the volume % of gas space made of the non-CO₂acid gas can be determined by dividing the partial pressure of a non-CO₂acid gas in the gas space by the total pressure of the gases in the gasspace and multiplying by 100%. As used herein, the term “gas space”refers to a space or a volume occupied by gas in a vessel (e.g., anadsorber vessel, a desorber vessel). In some embodiments, the gas spacein a desorber vessel is at the same pressure as the gas space in anadsorber vessel (further described elsewhere herein). In otherembodiments, the gas space in a desorber vessel at a different (e.g.,lower) pressure than the gas space in an adsorber vessel.

In some embodiments, the gas space in a vessel, in a system configuredfor batch operation (further described elsewhere herein), during aregeneration step comprises non-CO₂ acid gas, e.g., greater than orequal to 0.0001 volume % of the gas space in the vessel is made ofnon-CO₂ acid gas(es). In some embodiments, the gas space in a vessel, ina system configured for batch operation, during a regeneration step isat the same pressure as the gas space in the vessel during asequestration step. In other embodiments, the gas space in a vessel, ina system configured for batch operation, during a regeneration step isat a different (e.g., lower) pressure than the gas space in the vesselduring a sequestration step.

According to certain embodiments, a method comprises cycling the sorbentat least 2 (or at least 5, at least 10, at least 50, at least 100, atleast 1000, or at least 10,000) times. In some such embodiments, duringeach of the 2 (or during each of the 5, each of the 10, each of the 50,each of the 100, each of the 1000, and/or each of the 10,000)sequestration steps of the cycles, the amount of non-CO₂ acid gas thatis sequestered by the sorbent is at least 75%, at least 90%, at least95%, at least 98%, at least 99%, or at least 99.9% of the amount ofnon-CO₂ acid gas that is sequestered by the sorbent during an equivalentsequestration step of the 1^(st) cycle. In some such embodiments, duringeach of the 2 (or during each of the 5, each of the 10, each of the 50,each of the 100, each of the 1000, and/or each of the 10,000)regeneration steps of the cycles, the amount of non-CO₂ acid gas that isreleased by the sorbent is at least 75%, at least 90%, at least 95%, atleast 98%, at least 99%, or at least 99.9% of the amount of non-CO₂ acidgas that is released by the sorbent during an equivalent regenerationstep of the 1^(st) cycle. In some such embodiments, the amount ofnon-CO₂ acid gas that is released by the sorbent during the regenerationstep of the 1^(st) cycle is at least 75%, at least 90%, at least 95%, atleast 98%, at least 99%, or at least 99.9% of the amount of non-CO₂ acidgas that is sequestered by the sorbent during the sequestration step ofthe 1^(st) cycle. In some such embodiments, the amount of non-CO₂ acidgas that is sequestered during the sequestration step of the 1^(st)cycle, the 10^(th) cycle, and/or the 100^(th) cycle is at least 0.01mmol, at least 0.1 mmol, at least 0.5 mmol, at least 2.0 mmol, or atleast 10.0 mmol (and/or at most 20.0 mmol, at most 18.0 mmol, at most16.0 mmol, at most 14.0 mmol, or at most 12.0 mmol) per gram of thesorbent. In certain embodiments, the temperature of the sorbent duringthe sequestration/regeneration cycles is at most 1000° C., at most 850°C., at most 600° C., at most 550° C., or at most 520° C. (and/or atleast 200° C., at least 300° C., at least 400° C., at least 450° C., orat least 500° C.). In certain embodiments, the time over which each ofthe sequestration steps and each of the regeneration steps occurs is 24hours or less (or 12 hours or less, 8 hours or less, 4 hours or less, 1hour or less, 30 minutes or less, 10 minutes or less, or 2 minutes orless, and/or at least 10 seconds, at least 20 seconds, at least 30seconds, or at least 1 minute). In some embodiments, the steady stateconcentration of non-CO₂ acid gas in the environment to which thesorbent is exposed during the sequestration steps of thesequestration/regeneration cycles is as little as 50 mol %, as little as25 mol %, as little as 10 mol %, or as little as 1 mol % acid gas (e.g.,with the balance of the environment being argon or remaining acid gasespresent after sequestration of a certain acid gas).

In some embodiments, systems for sequestering non-CO₂ acid gas using asorbent comprising a salt in molten form are provided. Systems describedherein may be used to carry out methods described herein using sorbentsdescribed herein.

In some embodiments, a system configured for sequestering non-CO₂ acidgas in a batch operation is provided. A system configured for batchoperation may comprise any of a number of suitable components. In someembodiments, a system configured for batch operation comprises an inletto a vessel, the vessel, and an outlet to the vessel. In someembodiments during a sequestration step, the inlet is configured toreceive a fluid rich in non-CO₂ acid gas, which fluid can flow from theinlet to the vessel. In certain embodiments, the vessel is configured tocontain a sorbent described herein. In some embodiments during aregeneration step, the outlet is configured to receive fluid lean innon-CO₂ acid gas from the vessel, having a lower mole percentage of thenon-CO₂ acid gas than the non-CO₂ acid gas-rich fluid, at least becausesome sequestration by the sorbent occurred in the vessel. In someembodiments during a regeneration step, the inlet is configured toreceive energy or work (e.g., from a heated and/or pressured gas), whichcan flow from the inlet to the vessel. In some embodiments during aregeneration step, the outlet is configured to receive non-CO₂ acid gasfrom the vessel, due to regeneration of the sorbent in the vessel.

In some embodiments, a system configured for sequestering non-CO₂ acidgas in a continuous operation is provided. A system configured forcontinuous operation may comprise any of a number of suitablecomponents. In some embodiments, a system configured for continuousoperation comprises an inlet to an adsorber vessel, the adsorber vessel,and an outlet to the adsorber vessel. In some embodiments, a systemconfigured for continuous operation further comprises an inlet to adesorber vessel, the desorber vessel, and an outlet to the desorbervessel. In some embodiments, a system for continuous operation furthercomprises a first conduit between the adsorber vessel and the desorbervessel configured to transport a sorbent loaded with non-CO₂ acid gasfrom the adsorber vessel to the desorber vessel. In some embodiments, asystem further comprises a first pump configured in line with the firstconduit to transport the loaded sorbent. In certain embodiments, thefirst pump is a slurry pump. In some embodiments, a system forcontinuous operation further comprises a second conduit between theadsorber vessel and the desorber vessel configured to transport unloadedsorbent from the desorber vessel to the adsorber vessel. In someembodiments, a system further comprises a second pump configured in linewith the second conduit to transport the unloaded sorbent. In someembodiments, a system further comprises a heat exchanger in line withthe first conduit and/or second conduit (e.g., configured for atemperature swing operation). In some embodiments, a system furthercomprises a re-boiler or heater fluidically connected with the desorbervessel and the pump (e.g., configured for temperature swing operation).In some embodiments, a system further comprises a compressor fluidicallyconnected with the desorber vessel configured to output a pure acid gasstream. Systems provided herein may comprise any suitable combination ofcomponents.

Systems that are a hybrid of a system configured for batch operation anda system configured for continuous operation are also contemplated.

As used herein, “loaded” sorbent refers to sorbent at least a portion ofwhich (e.g., between or equal to 1 wt % and 90 wt %) has sequesterednon-CO₂ acid gas.

As used herein, “unloaded” sorbent refers to sorbent at least a portionof which (e.g., between or equal to 75 wt % and 100 wt %, between orequal to 85 wt % and 100 wt %, between or equal to 95 wt % and 100 wt %)has had a non-CO₂ acid gas removed.

In some embodiments, a system (e.g., a system for batch operation, asystem for continuous operation) provided herein includes at least onetemperature controller configured to control the temperature of a vessel(e.g., an adsorber vessel, a desorber vessel). For example, atemperature controller may be used to set the temperature of the vesselat or above the melting temperature of the salt of the sorbent, in orderto maintain at least some of the salt in molten form duringsequestration.

Systems (e.g., a system for batch operation, a system for continuousoperation) described herein can be used for a pressure swing non-CO₂acid gas separation operation at a high temperature (e.g., between orequal to 200° C. and 1000° C., between or equal to 500° C. and 700° C.)using a sorbent described herein. For example, in some embodiments,during a sequestration step (e.g., in an adsorber vessel), the partialpressure of non-CO₂ acid gas in a first environment to which the sorbentis exposed is between or equal to 0.000001 bar and 20 bar (e.g., betweenor equal to 0.1 bar and 10 bar), and the total pressure of the firstenvironment to which the sorbent is exposed is between or equal to 1 barand 30 bar, or between or equal to 1 bar and 50 bar. In someembodiments, the total pressure of the first environment to which thesorbent is exposed may be at least 1 bar, at least 2 bar, at least 5bar, at least 10 bar, at least 20 bar, at least 50 bar, at least 100bar, or more. In certain embodiments, during a sequestration step, thenon-CO₂ acid gas is between or equal to 1 ppm and 30 mol % of the firstenvironment (e.g., a stream). In some embodiments, during a regenerationstep (e.g., in a desorber vessel), the partial pressure of non-CO₂ acidgas in a second environment to which the sorbent is exposed is betweenor equal to 0 bar and 0.2 bar, and the total pressure of the secondenvironment to which the sorbent is exposed is between or equal to 1 barand 20 bar. In some embodiments, the total pressure of the secondenvironment to which the sorbent is exposed may be less than 20 bar,less than 10 bar, less than 5 bar, less than 2 bar, less than 1.5 bar,less than 1.2 bar, less than 1.1 bar, less than 1 bar, (e.g., undervacuum), less than 0.5 bar, less than 0.1 bar, or less than 0.01 bar. Insome embodiments, the difference between the total pressure of the firstenvironment and the total pressure of the second environment is betweenor equal to 0 bar and 20 bar. In certain embodiments, the differencebetween the total pressure of the first environment and the totalpressure of the second environment is at least 0.1 bar, at least 1 bar,at least 5 bar, at least 10 bar, at least 50 bar, at least 100 bar, ormore. In some embodiments, the difference between the partial pressureof non-CO₂ acid gas in the first environment and the partial pressure ofnon-CO₂ acid gas in the second environment is between or equal to 1 barand 20 bar. Other ranges are also possible. For example, in a pressureswing operation, a sorbent may be exposed to a first environment atpressure 30 bar with a partial pressure of non-CO₂ acid gas of 1 barduring a sequestration step, and the sorbent may be exposed to a secondenvironment at a pressure of 20 bar with a partial pressure of non-CO₂acid gas of 0 bar during a regeneration step.

Systems (e.g., a system for batch operation, a system for continuousoperation) described herein can be used for a temperature swing non-CO₂acid gas separation operation at a high base temperature (e.g., betweenor equal to 200° C. and 900° C.). For example, in some embodiments,during a sequestration step (e.g., in an adsorber vessel), a firsttemperature of a sorbent is held at between or equal to 200° C. and 900°C. In some embodiments, during a regeneration step (e.g., in a desorbervessel), a second temperature of a sorbent is held at between or equalto 250° C. and 950° C. In some embodiments, the difference between thesecond temperature and the first temperature is between or equal to 10°C. and 500° C. (e.g., between or equal to 20° C. and 400° C., 200° C.).For example, in a temperature swing operation, a sorbent may be held at500° C. during a sequestration step and 700° C. during a regenerationstep.

In certain embodiments, at least a portion of the sorbent containing atleast the portion of the non-CO₂ acid gas is removed from theenvironment. For example, in some embodiments (e.g., in some embodimentswhere the sorbent is not upconcentrated or regenerated), after thesorbent has captured non-CO₂ acid gas, the sorbent with captured acidgas may be discarded.

In some embodiments, after the sorbent has captured non-CO₂ acid gas, asolution can be added to the environment to precipitate at least aportion of the sorbent. In some such embodiments, the solution containscalcium ions. In certain embodiments, adding the solution results in theprecipitation of CaSO₄. As one example, in some embodiments, limewatercan be added to recover aqueous sorbent and gypsum.

U.S. Provisional Patent Application No. 62/971,488, filed Feb. 7, 2020,and entitled “Treatment of Acid Gases Using Molten Alkali Metal Borates,and Associated Methods of Separation,” is incorporated herein byreference in its entirety for all purposes.

The following example is intended to illustrate certain embodiments ofthe present invention but does not exemplify the full scope of theinvention.

Example

This example describes the removal of several non-CO₂ acid gases incomparison to CO₂ under both reducing and oxidizing environments.

An acid gas is any gas that forms an acidic solution with water. Thosemost relevant to industrial emissions are various oxides of sulfur(SO_(x)) and nitrogen (NO_(x)), hydrogen sulfide (H₂S), and carbondioxide (CO₂). Typically, acid gases are environmental pollutants, suchas greenhouse gases or producers of acid rain, and are, in some cases,severely harmful to human health. Recent interest in capturing CO₂emissions to combat global warming, masks an older effort to treat acidgas emissions more broadly, such as non-CO₂ acid gases. Many lessons canbe learned from these successes, both in terms of strategies for thelarge-scale deployment of emission control technology, and from thespecific technological challenges that were overcome. Indeed, many oftoday's best options for carbon capture trace their roots to thetreatment of other acid gases (e.g., non-CO₂ acid gases). For example,it was originally desirable for amines to remove H₂S, but not CO₂, innatural gas processing.

The method of capture may be the same in each case, in a sorber the acidgas is contacted with a basic sorbent to form a neutral salt, which istypically destabilized in a desorber by a change in conditions, forexample temperature, with the recovered gas sent for further treatment,storage, or utilization. The basicity of the various sorbents has beentuned over decades to target specific acid gases and lately, among otherprocess challenges, to minimize the energy penalty of the release step.Real systems present both a challenge and an opportunity in that theycontain multiple acid gases at varying concentrations. The opportunityis to treat multiple acid gases simultaneously thereby reducingequipment costs and system complexity. The challenge is to manage theproducts and maintain ever stricter limits on acid gas emissions. Arecent example of this trend can be seen in the shipping industry, whereSO_(x) emissions are being more tightly controlled from 2020 and theindustry aims to cut 70% of CO₂ emissions by 2050.

In understanding the relevance of the various acid gases to specificindustrial processes, it is convenient to distinguish between anoxidizing atmosphere, where an excess of oxidizing agent exists, and areducing atmosphere, where no such oxidizing agent exists. Iffeedstock's containing sulfur are processed under an oxidizingenvironment sulfur is emitted in the form of SO_(x), of which SO₂ is themost pertinent. Common examples include the combustion of coal, oil,natural gas, and, biomass, production of cement, and the smelting ofores. On the other hand sulfur forms H₂S in reducing environments, suchas those associated with pre-combustion technologies, hydrogenproduction, and gas-sweetening. Similarly, nitrogen is present in manyfeedstock's giving rise to fuel NO_(x), of which NO₂ and NO are the mostrelevant. In addition, nitrogen is present in the air making thermalNO_(x) emissions particularly pervasive in oxidizing environments.However, in some embodiments, under a reducing atmosphere nitrogen isusually emitted rather than NO_(x). For various high temperature sourcesTable 1, below, presents the typical range of uncontrolled acid gasemissions.

TABLE 1 Typical acid gas concentration by source. CO₂, SO_(x), andNO_(x) correspond to post-combustion designs while the H₂S concentrationcorresponds to pre-combustion. CO₂ SO_(x) H₂S NO_(x) Coal 12-14%100-4000 ppm 0.2-3%  100-800 ppm Oil 11-13% 100-4000 ppm 0.2-3%   50-700ppm Gas  3-4%   10-1000 ppm 10-1000 ppm   10-200 ppm Biomass  3-8%   10-200 ppm  50-300 ppm   50-400 ppm Cement 14-33%   3-1200 ppm N/A100-1500 ppm Metals 15-27%  150-400 ppm N/A  150-300 ppmHigh temperature capture, typically around 600° C. or 700° C., may offermany advantages over lower temperature operation including greateropportunities for efficient heat recovery, so-called “sorption enhanced”designs, and different chemistries with faster kinetics and highercapacities. The recent discovery of molten alkali metal borates(A_(x)B_(1-x)O_(1.5-x)) as high temperature liquid phase sorbents forcarbon capture, in some embodiments, represents a significant advance inrealizing efficient low-cost carbon capture facilities. Without wishingto be bound by any theory, two distinct reaction mechanisms have beenidentified, one where the molten liquid reacts to form solid crystallineproducts and another where the molten liquid reacts to form moltenliquid products. In some embodiments, the alkali metal borates withcompositions, Na_(x)B_(1-x)P_(1.5-x) (x=0.75) and(Li_(0.5)Na_(0.5))_(x)B_(1-x)O_(1.5-x) (x=0.75) are the representativeexamples of “liquid-to-solid” and ‘liquid-to-liquid’ type sorbents,respectively.

As described herein, the interaction between other acid gases (e.g.,non-CO₂ acid gases) and this new class of high-temperature sorbents arediscussed with a focus on the opportunity-challenge presented by realsystems. In some embodiments, the performance of molten alkali metalborates at the concentrations and mixtures applicable to real systemsusing SO₂ as a representative example are described, then the reactionmechanism for the industrially significant gases are described, andfinally the implications on the design of high-temperature capturefacilities with a comparison between the state-of-the-art in carboncapture and the molten alkali metal borates are described further below.

Experimental & Methodology Sample Preparation

Lithium hydroxide (LiOH, 98%), sodium hydroxide (NaOH, 97%), and boricacid (H₃BO₃, 99.5%) were purchased from Sigma-Aldrich. The alkali metalborate samples, A_(x)B_(1-x)O_(1.5-x), where A is an alkali metal and xis the mixing ratio, were prepared from mixed precipitants of alkalimetal hydroxide and boric acid. The mixtures were weighed and dissolvedin 0.1 g/mL Milli-Q (Millipore) deionised water. The water wasevaporated at 120° C. for several hours followed by 2 hours at 400° C.to release residual moisture and CO₂; a final pretreatment step at 800°C. was conducted for 60 minutes in-situ to obtain the targetedcomposition.

Performance Analysis

Mixtures of 1 mol % acid gas, balance nitrogen (N₂), were obtained(Airgas) for carbon dioxide (CO₂), sulfur dioxide (SO₂), hydrogensulfide (H₂S), and nitrogen dioxide (NO₂). 20% CO₂ was mixed with 1 mol% acid gas to obtain various mixtures of 10 mol % CO₂ & 0.5 mol % acidgas. The performance of the sorbents was analyzed by the weightvariation of the sample inside a thermogravimetric analyzer (TGA, Q50 TAInstruments) on exposure to a flow of gas. In each case, the sample masswas ˜5 mg and sample gas flow rate ˜30 mL/min. The final pretreatmentstep was carried out inside the TGA under 200 mL/min N₂. The weightchange on exposure to acid gas was normalized by the sample mass afterfinal pretreatment to obtain the loading in mg of gas per gram ofsorbent, in some cases for better comparison the weight change wasconverted into mmol of acid gas using the molecular weight of the gas.Temperature ramps were carried out after final pre-treatment from 200°C. to 800° C. at 5° C./min.

Materials Characterization

The phase composition and crystallographic features were examined bypowder X-ray diffractometry (XRD) and high temperature powder X-raydiffractometry (HTXRD) (XRD: PANalytical X'Pert Pro MultipurposeDiffractometer with Cu-k_(α) X-ray (Xλ=1.541 Å)), in which the sampleswere placed on a Pt-sheet substrate. The peaks in the XRD spectra wereidentified by referring to the ICDD PDF-4+2016RDB database. Samples wereprepared ex-situ in a tube furnace (GSL-1800, MTI Corp), under acontinuous flow of 1 mol % acid gas balance N₂ for 60 minutes.

Results and Discussion

From Table 1, above, some of the industrially relevant acid gasconcentrations can vary by orders of magnitude. In this example, SO₂ isselected as a representative species at the higher end of concentrations(1 mol %, 0.5 mol %, and 0.1 mol %) where the acid gases influence wouldbe most significant, and comparisons are drawn to CO₂ capture at thesame concentration.

At 600° C., the performance of the sodium borate Na_(x)B_(1-x)O_(1.5-x)(x=0.75) for SO₂ capture was similar to CO₂ capture on a molar basis(FIG. 3A). For an acid gas concentration of 1 mol % and 0.5 mol % thecapacity was consistently ˜6 mmol/g but, 0.1 mol % was too low for thereaction to reach completion in 60 minutes. However, there was a clearinteraction between the acid gases and the sorbent even at this lowconcertation. Exposure to nitrogen in the release step stimulatedpressure swing operation by reducing the partial pressure of the acidgas. At 600° C. CO₂ was very slowly released whereas SO₂ was retained bythe sorbent with a slight increase in loading. As the acid gasconcentration dropped from 0.1 mol % to 0% during exposure to nitrogenthe sorbent continued to remove any left-behind SO₂ still present in thehead space, suggesting concentrations substantially less than 0.1 mol %could be treated effectively.

At 700° C. (FIG. 3B), the capacity for CO₂ was reduced at lowerconcentrations as CO₂ existed in the melt below supersaturationconditions, while the capacity for SO₂ was elevated. Without wishing tobe bound by any theory, the elevated SO₂ loading suggested a differentreaction occurred that allowed for a higher capacity at 700° C. comparedto 600° C. Release of CO₂ was more favorable at 700° C. but in each caseSO₂ was retained at the capacity reached during the uptake step. Thissuggests that while the reaction with CO₂ is easily reversed thereaction with SO₂ is irreversible up to at least 700° C.

Likewise for the lithium-sodium borate(Li_(0.5)Na_(0.5))_(x)B_(1-x)O_(1.5-x) (x=0.75) (FIG. 3C). At 600° C.the initial reaction rate closely matched that of CO₂ but the capacitywas substantially higher. For the lithium-sodium borate, the CO₂ in themelt was in equilibrium with the CO₂ in the gas stream resulting in agradual decrease in capacity with CO₂ concentration. Similar to thesodium borate, for SO₂ the same −11 mmol/g was reached for both 1 mol %and 0.5 mol %, 60 minutes under 0.1 mol % was insufficient for thecomplete reaction to take place. Also, the reaction with CO₂ wasreversed on exposure to nitrogen but SO₂ was retained.

At 700° C. (FIG. 3D) the capacity for CO₂ was lower as the releasereaction became more favorable while for SO₂ a similar uptake profilewas observed but with a second slower process occurring at high loadingsresulting in a gradual increase beyond that seen at 600° C. Again,without wishing to be bound by any theory, the release profile at 700°C. confirms the irreversibility of SO₂ uptake with both sodium andlithium-sodium borate.

Acid Gas Mixtures

In many real systems, multiple acid gases exist together and withsocieties growing environmental awareness it is expected that futurefacilities will require tight control of all acid gas emissions.Therefore, a mixture of 10 mol % CO₂ and 0.5 mol % SO₂ was examined,which resembles a worst-case scenario for a power plant burning highsulfur bituminous coal.

To gain an understanding of the influence of each acid gas and possibleinteractions between the two, a number of uptake experiments wereperformed including each gas individually, 10 mol % CO₂ (CO₂) and 0.5mol % SO₂ (SO₂), and both gases together 10 mol % CO₂ and 0.5 mol % SO₂(CO₂ & SO₂). In some variants these were followed by a change to anothermixture of acid gases. Such as, 10 mol % CO₂ followed by 0.5 mol % SO₂(CO₂→SO₂), 0.5 mol % SO₂ followed by 10 mol % CO₂ (SO₂→CO₂), andfinally, 10 mol % CO₂ followed by the mixture of 10 mol % CO₂ and 0.5mol % SO₂ (CO₂→CO₂ & SO₂). The difference between (CO₂→SO₂) and (CO₂→CO₂& SO₂) is that in the former the partial pressure of CO₂ changes whereasin the latter it does not. Subsequently the sorbent was exposed tonitrogen in the release step, in the cases without a displacement step adashed line connects the uptake and release profiles. The loading isreported on a mass rather than molar basis since in the case of mixturesit cannot be known for certain which gas reacted and hence whichmolecular weight to apply.

For the sorbent Na_(x)B_(1-x)O_(1.5-x) (x=0.75) at 600° C. (FIG. 4A),CO₂ rapidly reacted and reached full capacity within just a few minutes.The concentration of SO₂ was 20 times lower so the reaction was slowerfor SO₂ individually, but the loading approached the full capacity seenin FIG. 3A within 60 minutes. For the mixture of both gases the loadingclosely matched that of CO₂ initially but then continued to increase.However the capacity of SO₂ individually was not reached with 60minutes, suggesting that CO₂ was being slowly displaced by SO₂. The samewas true when the sorbent was first loaded with CO₂ and then exposed toSO₂, and when loaded with CO₂ and then exposed to the mixture. Thesethree cases were similar because at 600° C. the change in partialpressure of CO₂ does not result in significant release of CO₂. Underrelease conditions at 600° C. desorption was not favorable for eithergas. CO₂ loading dropped slightly but most other variations resulted inno significant release. Without wishing to be bound by any theory, inthe case of SO₂→CO₂ the loading remained approximately constantconfirming that SO₂ reacts irreversibly with the sorbent and cannot bedisplaced by CO₂ in some embodiments.

The results at 700° C. differed from those at 600° C. partly because therelease of CO₂ is favorable at the higher temperature. In FIG. 4B, themixture of gases first follows the path of CO₂ individually thendeviates and approaches the capacity of SO₂ individually. In the case ofCO₂→SO₂ a slight drop in loading was seen as CO₂ was released before theloading increased with the uptake of SO₂. In some cases, the rate ofuptake of SO₂ is faster for CO₂→SO₂ than the cases SO₂ & CO₂ and CO₂→CO₂& SO₂. The presence of CO₂ slows the displacement reaction as thecarbonate product remains stable. As seen at 600° C. CO₂ does notdisplace SO₂ in the SO₂→CO₂ variation. Under release conditions CO₂individually was quickly released while SO₂ was not released. The changein loading for each variation therefore indicates the relativeproportion of CO₂ and SO₂ captured.

The lithium-sodium borate (Li_(0.5)Na_(0.5))_(x)B_(1-x)O_(1.5-x)(x=0.75) differed from the sodium borate in that the capacity wasgreater, the CO₂ reaction was reversible at at least some temperatures,and the reaction products were liquids. At 600° C., FIG. 4C, the mixturefirst tracks the loading of CO₂ individually but then quickly displacesthis CO₂ with SO₂ and tracks the loading of SO₂ individually. Withoutwishing to be bound by any theory, it is believed that the carbonate ionstabilized in the melt through coordination with free lithium and sodiumions and were more easily displaced than the solid sodium carbonatecrystals in the case of the sodium borate. Therefore, each variation thecapacity reached that of SO₂ individually regardless of the presence ofCO₂ or original loading. The complete displacement of CO₂ by SO₂ wassupported by the release profiles in which show the release of CO₂individually but no release for any other variant. Without wishing to bebound by theory, it is believed that at 700° C. (FIG. 4D), the capacityfor CO₂ decreased due to more favorable release conditions but otherwisethe profiles for each variation are similar to those at 600° C.

Oxides of Sulfur, SO_(x)

FIG. 3A suggests that for Na_(x)B_(1-x)O_(1.5-x) (x=0.75) at 600° C. thereaction proceeded in a similar manner for both SO₂ and CO₂. It is knownthat for CO₂ the general reaction proceeds with conversion to sodiummetaborate (NaBO₂, x=0.50) and sodium carbonate (Na₂CO₃),

$\begin{matrix}{\left. {{\left( \frac{1}{x - {0.5}} \right){Na}_{x}B_{1 - x}O_{{1.5} - x}} + {CO}_{2}}\rightarrow{{\left( \frac{1 - x}{x - {0.5}} \right){NaBO}_{2}} + {{Na}_{2}{CO}_{3}}} \right.,\mspace{79mu}{{{where}\mspace{14mu} 0.50} < x < 1}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

For the case where this initial composition of the sodium borate isx=0.75 the stoichiometry of the reaction with conversion to x=0.50 canbe written as,

Na₃BO₃+CO₂→NaBO₂+Na₂CO₃  Equation 2

The similarity between the uptake profiles at 600° C. implies the SO₂reaction is analogous,

Na₃BO₃+SO₂→NaBO₂+Na₂SO₃  Equation 3

Assuming this is true, the complete reaction should correspond to acapacity of 501 mg/g. FIG. 5A, shows the loading as a function oftemperature for Na_(x)B_(1-x)O_(1.5-x) (x=0.75) under 1 mol % SO₂ with agradual temperature ramp of 5° C./min. In addition the capacity underisothermal uptake conditions from FIG. 3, above, is represented by dotsand the dashed lines show the theoretical capacity for a given reaction,for example (x=0.50, Na₂SO₃) corresponds to Equation 3, where the boratereacts to x=0.50 (NaBO₂) and the sulfurous product Na₂SO₃. Thedifference between the temperature ramp and the isothermal capacity isdue to relatively slow kinetics under 1 mol % SO₂.

FIG. 5A, shows the capacity at 600° C. was slightly below but close tothat predicted by Equation 3 supporting the view that this is theprimary reaction mechanism. In addition, XRD analysis, FIG. 5B, revealedthe dominant peaks corresponded to sodium metaborate (NaBO₂) and sodiumsulfite (Na₂SO₃). However, at 700° C. the isothermal capacity exceededthis capacity which cannot be explained by Equation 3. Without wishingto be bound by any theory, a possibility is the occurrence of thefollowing decomposition reaction which has been known to occur at around700° C.,

4Na₂SO₃→3Na₂SO₄+Na₂S  Equation 4

Equation 4 explains the observation of sodium sulfate (Na₂SO₄) ratherthan sodium sulfite (Na₂SO₃) by XRD after reaction at 700° C., FIG. 5B.To explain the increased capacity, without wishing to be bound by anytheory, consider that sodium sulfide, Na₂S, may have reacted furtherwith SO₂ with the involvement of platinum, from the platinum pan orplatinum substrate, to generate platinum sulfide (PtS) and more sodiumsulfate (Na₂SO₄),

Na₂S+2SO₂+2Pt→2PtS+Na₂SO₄  Equation 4B

Without wishing to be bound by any theory, this reaction could raise thecapacity to ˜630 mg/g and provide an explanation for the slower secondincrease in loading observed in the uptake experiments, FIG. 3B,furthermore platinum sulfide peaks observed by XRD after reaction at700° C., FIG. 5B.

Without wishing to be bound by any theory, similar reactions might alsoexplain the performance of the lithium-sodium borate, FIG. 5C. However,at 600° C. the capacity exceeded that predicted by the equivalent ofEquation 3 suggesting that at both temperatures considered theequivalent of Equation 4 played an important role. Without wishing to bebound by any theory, the observation of capacity even greater thanpredicted by Equation 4 for the lithium-sodium borate under isothermaluptake at 700° C. could be due to the formation of poly-sulfides or theconversion of lithium borate to compositions less than x=0.50, forexample x=0.25 (Li₂B₄O₇), which has been proposed as a possible productof the reaction between tri-lithium borate (Li₃BO₃) and CO₂.

XRD at 25° C. after reaction at 600° C., FIG. 5D, supports the reactionto sulfate products with peaks corresponding to sodium sulfate (Na₂SO₄),lithium-sodium sulfate (LiNaSO₄), and lithium metaborate (LiBO₂). Theexpected instability of lithium sulfite (Li₂SO₃), and the absence of anysodium borate peaks suggest that sodium is the dominant alkali metal inthe reaction with SO_(x). Without wishing to be bound by theory, it isthought that Equation 3 and Equation 4 both occur as written with sodiumtaking part in the reactions. Subsequently sodium sulfate forms and someof the lithium present in the melt coordinates with the sulfate to formlithium-sodium sulfate.

Sodium sulfate melts at −880° C., which suggested forNa_(x)B_(1-x)O_(1.5-x) (x=0.75) the SO₂ reaction mechanism is similar toCO₂, in that the gas reacted with the liquid sorbent to form solidprecipitants. However, for the lithium-sodium borate, in someembodiments, this may not be the case. With CO₂ the lithium-sodiumborate, (Li_(0.5)Na_(0.5))_(x)B_(1-x)O_(1.5-x) (x=0.75), forms aeutectic such that both the borate and the carbonate products are in theliquid phase above ˜500° C. In the case of SO₂ HTXRD showed that at 600°C. no peaks corresponding to an alkali metal borate were present,suggesting these species exists in the liquid phase. Only thelithium-sodium sulfate (LiNaSO₄) phase remained a solid crystal at ˜600°C., but at 700° C. this compound was also brought into the melt suchthat all reaction products with SO₂, with the exception of platinumsulfide, were liquids at 700° C., similar to the case with CO₂.

The reactions and mechanisms proposed in this example may have importantramifications on the design of high-temperature carbon capturefacilities and will be discussed further below. However, a discussion isincluded here of two expected phenomena of interest in the oxidizingenvironment of real exhausts containing SO_(x). First, without wishingto be bound by theory, sodium sulfide is unlikely to be stable in thepresence of an oxidizing agent, for example,

Na₂S+2O₂→4Na₂SO₄  Equation 5

Hence, corrosion of the reactor vessel by sulfurization may be lesslikely than implied by the formation of transition metal sulfide in theabove examples. And second, at high temperatures SO₂ may react withoxidizing agents to SO₃, which the latter typically comprises 0.1 to 3mol %,

SO₂+½O₂< >SO₃  Equation 6

Although not studied explicitly mentioned in this example, sulfurtrioxide (SO₃) is contemplated to be efficiently captured by a moredirect reaction with the molten alkali metal borates (A₃BO₃) to formsulfates, (A₂SO₄) bypassing the formation of sulfites (A₂SO₃) andsulfides (A₂S),

A₃BO₃+SO₃→ABO₂+A₂SO₄  Equation 7

Hydrogen Sulfide, H₂S

Hydrogen sulfide presents an interesting parallel to the oxides ofsulfur. As with SO_(x) the interaction between the basic sorbent and theacidic gas was strong, which may result in the efficient removal of thegas. FIG. 6A shows the weight change of the sodium borateNa_(x)B_(1-x)O_(1.5-x) (x=0.75) under a temperature ramp of 5° C./minand 1 mol % H₂S. Note that, in some cases, limited experiments werecarried out with H₂S due to its propensity to damage the instrumentationused in the uptake experiments. However, the results for the sodiumborate may be sufficient to give some useful insight into the reactionmechanism.

Unlike with CO₂ and SO₂, the reaction with H₂S began at ˜430° C. muchlower than the melting point of the sorbent (˜570° C.). Between 550° C.and 650° C. the loading plateaus around 350 mg/g before increasingfurther as the temperature was increased. The typical reaction betweenmetal oxides and H₂S involves the formation of metal sulfides andwater/steam, for example,

Na₃BO₃+H₂S→NaBO₂+Na₂S+H₂O  Equation 8

Without wishing to be bound by any theory, the theoretical capacity ofthis reaction is only 126 mg/g, which was quickly surpassed in theuptake experiment, but even complete conversion to x=0, with reactionproducts Na₂S and B₂O₃, would only give a theoretical capacity of 283mg/g. However, the formation of poly-sulfides or sodium platinum sulfide(Na₂PtS₂) with conversion of the alkali metal borate to x=0.50 mayexplain the loading in the range 550° C. to 650° C., for example,

Na₃BO₃+2H₂S→Na₂PtS₂+H₂O+H₂+NaBO₂  Equation 9

which was supported by XRD, FIG. 6B, revealing peaks that after reactionat 600° C. can ascribed to Na₂S, NaBO₂, and Na₂PtS₂. In certainembodiments above 650° C., the reaction may proceed to a conversion lessthan x=0.50, at the extreme x=0, which would give a theoretical capacityof ˜1700 mg/g. In some cases, no uptake experiments were carried outwith the lithium-sodium borate (Li_(0.5)Na_(0.5))_(x)B_(1-x)O_(1.5-x)(x=0.75); however, XRD revealed a reaction similar to Equation 9 islikely with products including Na₂PtS₂ and Li₆B₄O₉.Oxides of Nitrogen, NO_(x) The oxides of nitrogen present a complex andchallenging case to study. Firstly, gas phase chemistry plays anespecially important role as at the high temperatures of interest NO₂decomposes by the equilibrium reaction,

NO₂↔NO+½O₂  Equation 10

resulting in a mixture that is only 5 to 10 mol % NO₂ at captureconditions. Secondly, the expected nitrate and nitrite products bringcomplexity in the multiple consecutive and reversible reactions that mayoccur. Thirdly, as with H₂S, in some embodiments, limited experimentscould be carried out with NO_(x) due to damage inflicted on equipment.Despite these challenges, meaningful conclusions can be drawn from theinteraction between the molten alkali metal borates and NO_(x) emittedfrom high temperature sources.

FIG. 7A shows the weight change of the sodium borateNa_(x)B_(1-x)O_(1.5-x) (x=0.75) under a temperature ramp of 5° C./minand 1 mol % NO₂. Uptake began as low as 300° C. and reached a peak at600° C. before release becomes favorable and the loading decreases, at˜700° C. a negative loading was observed, indicating a loss of mass fromthe original sodium borate. Without wishing to be bound by any theory,sodium nitrate and nitrite are liquids above ˜300° C., and may catalyzetheir own reaction by providing a liquid surface capable of reactingrapidly with the solid sodium borate at temperatures much lower than itsmelting point, as is the case in molten nitrate promoted CO₂ captureusing metal oxides. Decomposition and vaporization of the nitrates andnitrates is known to occur simultaneously at temperatures above 600° C.,which could explain the loss of mass above ˜700° C. Based on theoreticalcapacity, the reaction with NO_(x) appears to have the stoichiometryclose to,

Na₃BO₃+NO₂+NO→NaBO₂+2NaNO₂  Equation 11

though it is recognized that in practice it is likely a large number ofgas and liquid phase reactions lead to the net reaction given byEquation 11.

The observation of peaks corresponding to sodium nitrite (NaNO₂) andsodium metaborate (NaBO₂) after exposure to NO₂ at 600° C., FIG. 7B,supported this net reaction for certain embodiments. However, in someembodiments, unreacted tri-sodium borate (x=0.75), and partially reacteddi-sodium borate (x=0.66) were also observed indicating that thereaction did not proceed to the same extent in the tube furnace. Afterreaction at 800° C., the peaks ascribed to sodium nitrite (NaNO₂) wereno longer present leaving a mixture of tri-sodium borate (Na₃BO₃) anddi-sodium borate (Na₄B₂O₅). Decomposition to Na₄B₂O₅ has a theoreticalmass loss of 243 mg/g, therefore the mixture of Na₃BO₃ and Na₄B₂O₅,which may explain the observed loss of ˜150 mg/g. The decomposition ofsodium nitrite involves a number of reactions including the evolution ofgaseous N₂, O₂, and NO, and solid Na₂O, which would recombine with theborate melt, in addition to vaporization.

Isothermal uptake and release were demonstrated for some embodiments inFIG. 7C at 600° C. without the mass loss associated with highertemperatures. As with other acid gases the behavior of lithium-sodiumborate was similar to sodium borate, as demonstrated by the temperatureramp in FIG. 7D. The maximum loading was lower, and the reaction did notgo to completion, but significant uptake was observed even at 1 mol %NO₂. Without wishing to be bound by any theory, as withsulfates/sulfites, the lithium nitrates/nitrates were less stable thantheir sodium counterparts making it likely that sodium was the dominantalkali metal involved in the above reactions, with lithiumpreferentially interacting with the borate species.

The understating developed above may influence the design ofhigh-temperature carbon capture systems. The strong interaction betweenthe molten alkali metal borates and the acid gases demonstrate that asorber designed for carbon capture may capture not just CO₂ but anyother acid gases (e.g., non-CO₂ acid gases) present. This couldrepresent an opportunity to capture multiple acid gases in a singlesorber, or a challenge to manage various corrosive or hard-to-handlereaction products. The distinction between opportunity and challengelies in design, which will be the topic of this section of the Example.

One of the principal advantages of the molten alkali metal borates istheir fluidic nature, which may provide easy transfer between sorber anddesorber at high temperatures, as depicted in FIG. 8, for someembodiments. Without modification, considering the plots in FIG. 8, twooutcomes may be possible after capture of acid gases other than CO₂,either their release into the CO₂ product stream in the desorber, ortheir accumulation in the system until the sorbent needs to be replaced.Note that CO₂ is in excess in the sorber resulting in a relatively lowloading of other acid gases. Therefore, without wishing to be bound byany theory, it is not expected that their presence will significantlyinterfere with the pumping or fluidity of the bulk sorbent, unless theyaccumulate.

In many cases, the reaction with SO_(x) is irreversible at reasonabletemperatures; however, the reaction with NO_(x) was easily reversed uponpressure-swing. Therefore, sulfurous products will be retained after thedesorption step while products of the NO_(x) reaction will be releasedinto the CO₂ product stream. Gaseous release from NO_(x) products atless than 700° C. comprise N₂, O₂, and NO, which may be acceptable inthe product stream in small quantities. To reduce the vaporization ofnitrates/nitrites and hence loss of alkali metal from the system, it wasdesirable to maintain desorption temperatures less than 700° C. The lossof alkali metal from the system may slightly reduce the mixing ratio, x,of the molten alkali metal borate circulating between sorber anddesorber. However, the mixing ratio could be raised by alkali metalhydroxide/carbonate addition. If loses are small and predominantlysodium rather than lithium, which is expected, this approach can be bothsimple and cost effective.

A different strategy may be useful to handle sulfurous products as thesemay build-up in the system. One option would be to include a purgestream and continually replenish the sorbent. However, the nature of themolten alkali metal borates could allow for not only the simultaneouscapture of multiple acid gases but also their efficient separation.Since the molten alkali metal borates are liquids and the sulfurousproducts are typically solids any number of liquid-solid separationtechniques could be applied to a slip stream of the circulating sorbent,which would separate out the sulfurous compounds at high temperatures,as schematically depicted in FIG. 8.

As the density difference between the melt and the sulfurous products isrelatively small, filtration may outperform gravitational methods ofseparation. For example, cross-flow filtration with a metal meshseparator applied to a slip stream downstream of the desorber pump maybe a suitably simple and robust method at the high temperaturesinvolved. A large mesh size would reduce pressure drop and allow for amodest amount of solids recirculation. Once upconcentrated, thesulfurous compounds could be further refined and purified under ambientconditions and made into marketable products. Several options existincluding some at high temperatures which would minimize thermal loses.However, it is likely that the value of lithium relative to all otherspecies will dictate the treatment method. One option would be tocontact the concentrated sulfate stream with limewater, (Ca(OH)₂(aq)),as schematically illustrated in FIG. 8. Without wishing to be bound byany theory, in the aqueous phase at ambient temperatures the sulfurousspecies would precipitate calcium sulfate (CaSO₄), for example,

Ca(OH)_(2 (aq))+NaLiSO_(4 (aq))→CaSO_(4 (s))+NaOH_((aq))+LiOH_((aq))  Equation12

The resultant gypsum (CaSO₄.2H₂O) could be sold as is common practice inthe industry today, and the remaining ions in solution, which mayinclude some borate species but more importantly valuable lithium ions,could be returned to the high temperature system. Evaporation of thewater required for dissolution would leave predominantly alkali metalhydroxides which would return the sorbent to its original mixing ratio,x.

Assuming the reactor vessel can be protected from sulfurization, asimilar process to that described for SO_(x) may be appropriate forsimultaneous H₂S capture and separation. Solid Na₂S may be filtered andtreated. As with sulfates, treatment would depend on the desiredsulfurous product, but to ensure recovery of the alkali metal and boratespecies the reaction with oxygen to generate sulfates for subsequenttreatment may be preferable. In the case of a reducing environmentcomprising no oxidizing agent will be present to protect the reactorvessel from sulfuization, as was the case in the oxidizing environment.In existing facilities that handle H₂S, particularly at hightemperatures, sulfurization remains a major problem. Indeed, this is theprimary motivation for pre-treatment in refineries and gas-sweeteningfacilities. Sulfurization can be subdued by material selection and theformation of a thin protective metal-sulfide layer. This and othercorrosion prevention techniques could allow for the removal of H₂S bythe molten alkali metal borates without undesirable reaction with themetals present in the sorber vessel walls or packing. Having said this,it is contemplated that upstream desulfurization, for example naturalgas sweetening, will remain important to future carbon capturefacilities operating under a reducing environment.

Comparison with Others

The conceptual designs described elsewhere herein may resemble some ofthe existing strategies in use for amine and calcium looping systemswhich, in some cases, represent the state-of-the-art in low and hightemperature carbon capture, respectively. However, the molten alkalimetal borates present a number of advantages in the context of dealingwith acid gas impurities, which are briefly described below.

In certain embodiments, SO_(x) react with amines to form dissolvedsulfites/sulfates in the aqueous phase; however, the inability toupconcentrate these species requires that a large proportion of therecirculating amine solution be regularly purged for treatment. A numberof options exist, with thermal reclamation, amines evaporation andrecovery, being the most established. However, this approach is usuallyonly cost effective with upstream flue gas desulfurization, sincethermal reclamation results in amine losses, and presents a relativelylarge energy penalty and contaminates the semi-solid product.

In some embodiments, NO_(x) also react with amines, forming dissolvednitrites/nitrates and nitrosamines/nitramines. Depending on the aminethis absorption can result in modest removal of NO_(x) to near completeremoval. While these reactions may provide some benefit in NO_(x)reduction the net effect is generally negative due to the health risksassociated with some nitrosamines, which will be present in the solidproduct, and the relatively high cost of amines. As mentioned elsewhereherein, amines may be well suited to H₂S capture, in some cases, but notfor high-temperature applications as described herein, such aspre-combustion carbon capture and hydrogen production. For appropriatecomparison with a high-temperature sorbent, we consider calcium oxideand the calcium looping process.

Calcium oxide may also remove SO_(x) from the exhaust stream. However,in calcium looping, CaO, CaCO₃ and CaSO₄ are all solids and cannot beeasily separated, requiring the regular purging in of the system. Thesituation is worsened by calcium sulfates propensity to block internalpores in the solid sorbent, reducing its capacity for CO₂. Calciumcompounds pervasiveness and low cost partly make up for this shortcomingbut the outcome is not ideal. In reducing environments, calcium oxidereacts with hydrogen sulfide (H₂S) to form calcium sulfide (CaS), whichis also purged as the sorbent degrades. Without wishing to be bound byany theory, although calcium oxide has no propensity to react withNO_(x), the use of oxy-combustion to generate a large portion of theplants power results in marginally lower NO_(x) emissions when comparedto a reference power plant. However, these emissions would still beunacceptably high and downstream NO_(x) scrubbing would be required.

CONCLUSIONS

The interaction between the various acid gases (e.g., non-CO₂ acidgases) present and the sorbent designed for carbon capture is a keychallenge for the state-of-the-art of carbon capture, and a majorstumbling block for less mature technologies. In this example andelsewhere herein, it has been demonstrated that the molten alkali metalborates may largely overcome this pitfall and do so in a way that mayoutperform both amines and calcium looping. This is not to say thatchallenges do not exist. The importance of material selection,specifically the requirement that vessels and lines containing sulfidesbe resistant to attack by sulfurization has been demonstrated. It isnoted that excess oxygen is desirable in this regard as it will help tosuppress the reaction pathway to sulfide formation, as suchhigh-temperature reducing environments rich in H₂S should be minimized,in some cases, with upstream treatment as is common practice in industrytoday, for example natural gas sweetening. It is also noted thattemperatures be limited to ˜700° C. to minimize vaporization ofnitrate/nitrite species in some cases involving non-acid gas H₂S.

However, the net outcome leans heavily towards opportunity. The stronglybasic nature of the molten alkali metal borates mean that acid gases canbe removed at the low concentrations and mixtures relevant to realsystems. The fluidic nature of the molten alkali metal borates allowsfor designs that take advantage of the typically solid sulfurousproducts for their efficient separation at high temperatures. Whencompared to the options available to amines and calcium looping the,designs proposed for the molten alkali metal borates appear superior,bolstering the advantages already afforded to this new class of sorbentfor carbon capture. Indeed the molten alkali metal borates andassociated system designs and mehtods may prove sufficiently efficientto generalize carbon capture to the broader challenge of acid gascapture.

While several embodiments of the present invention have been describedand illustrated herein, those of ordinary skill in the art will readilyenvision a variety of other means and/or structures for performing thefunctions and/or obtaining the results and/or one or more of theadvantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the present invention.More generally, those skilled in the art will readily appreciate thatall parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the teachings of thepresent invention is/are used. Those skilled in the art will recognize,or be able to ascertain using no more than routine experimentation, manyequivalents to the specific embodiments of the invention describedherein. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, the invention maybe practiced otherwise than as specifically described and claimed. Thepresent invention is directed to each individual feature, system,article, material, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,and/or methods, if such features, systems, articles, materials, and/ormethods are not mutually inconsistent, is included within the scope ofthe present invention.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Other elements may optionallybe present other than the elements specifically identified by the“and/or” clause, whether related or unrelated to those elementsspecifically identified unless clearly indicated to the contrary. Thus,as a non-limiting example, a reference to “A and/or B,” when used inconjunction with open-ended language such as “comprising” can refer, inone embodiment, to A without B (optionally including elements other thanB); in another embodiment, to B without A (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of.” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” and the like are to be understoodto be open-ended, i.e., to mean including but not limited to. Only thetransitional phrases “consisting of” and “consisting essentially of”shall be closed or semi-closed transitional phrases, respectively, asset forth in the United States Patent Office Manual of Patent ExaminingProcedures, Section 2111.03.

1. A method, comprising: exposing a sorbent, the sorbent comprising asalt in molten form, to an environment containing a non-CO₂ acid gassuch that at least a portion of the non-CO₂ acid gas interacts with thesorbent and at least a portion of the non-CO₂ acid gas is removed fromthe environment, wherein the salt in molten form comprises an alkalimetal, boron, and oxygen.
 2. The method of claim 1, wherein the salt inmolten form comprises an alkali metal borate, M_(x)B_(1-x)O_(1.5-x),wherein M is one or more alkali metals, B is Boron, O is Oxygen, and xis a number such that 0<x<1.
 3. The method of claim 2, wherein thealkali metal comprises lithium (Li), sodium (Na), potassium (K), and/ora mixture of these.
 4. The method of claim 3, wherein the alkali metalcomprises Li and Na in equal amounts.
 5. The method of claim 1, whereinthe non-CO₂ acid gas comprises sulfur monoxide (SO), sulfur dioxide(SO₂), nitrogen dioxide (NO₂), hydrogen sulfide (H₂S), sulfur trioxide(SO₃), nitric oxide (NO), nitrous oxide (N₂O), dinitrogen trioxide(N₂O₃), dinitrogen tetroxide (N₂O₄), dinitrogen pentoxide (N₂O₅), and/orcarbonyl sulfide (COS).
 6. The method of claim 1, wherein theenvironment is at a temperature of at least 200° C.
 7. The method ofclaim 1, wherein a plurality of non-CO₂ acid gases interacts with thesorbent such that at least a portion of the plurality of non-CO₂ acidgases are removed from the environment.
 8. The method of claim 1,wherein captured non-CO₂ acid gas is released in the gas phase in aseparate environment.
 9. The method of claim 8, wherein the release isdriven, at least in part, by a change in partial pressure of the non-CO₂acid gas and/or a change in temperature in the separate environmentrelative to the other environment.
 10. The method of claim 1, whereincaptured non-CO₂ acid gas forms a solid suspended in the molten sorbent.11. The method of claim 10, further comprising physically separating thesolid from the molten sorbent using a cross-flow filter, centrifugation,crystallization, and/or sedimentation.
 12. The method of claim 11,wherein the physical separation uses a cross-flow filter, and thecross-flow filter is operated at a temperature of at least 200° C.13-16. (canceled)
 17. The method of claim 2, wherein the non-CO₂ acidgas comprises sulfur monoxide (SO), sulfur dioxide (SO₂), nitrogendioxide (NO₂), hydrogen sulfide (H₂S), sulfur trioxide (SO₃), nitricoxide (NO), nitrous oxide (N₂O), dinitrogen trioxide (N₂O₃), dinitrogentetroxide (N₂O₄), dinitrogen pentoxide (N₂O₅), and/or carbonyl sulfide(COS).
 18. The method of claim 2, wherein the environment is at atemperature of at least 200° C.
 19. The method of claim 2, wherein aplurality of non-CO₂ acid gases interacts with the sorbent such that atleast a portion of the plurality of non-CO₂ acid gases are removed fromthe environment.
 20. The method of claim 1, wherein the alkali metalcomprises lithium (Li), sodium (Na), potassium (K), and/or a mixture ofthese.
 21. The method of claim 20, wherein the alkali metal comprises Liand Na.
 22. The method of claim 20, wherein the non-CO₂ acid gascomprises sulfur monoxide (SO), sulfur dioxide (SO₂), nitrogen dioxide(NO₂), hydrogen sulfide (H₂S), sulfur trioxide (SO₃), nitric oxide (NO),nitrous oxide (N₂O), dinitrogen trioxide (N₂O₃), dinitrogen tetroxide(N₂O₄), dinitrogen pentoxide (N₂O₅), and/or carbonyl sulfide (COS). 23.The method of claim 21, wherein the non-CO₂ acid gas comprises sulfurmonoxide (SO), sulfur dioxide (SO₂), nitrogen dioxide (NO₂), hydrogensulfide (H₂S), sulfur trioxide (SO₃), nitric oxide (NO), nitrous oxide(N₂O), dinitrogen trioxide (N₂O₃), dinitrogen tetroxide (N₂O₄),dinitrogen pentoxide (N₂O₅), and/or carbonyl sulfide (COS).
 24. Themethod of claim 20, wherein the environment is at a temperature of atleast 200° C.
 25. The method of claim 21, wherein the environment is ata temperature of at least 200° C.
 26. The method of claim 20, wherein aplurality of non-CO₂ acid gases interacts with the sorbent such that atleast a portion of the plurality of non-CO₂ acid gases are removed fromthe environment.
 27. The method of claim 21, wherein a plurality ofnon-CO₂ acid gases interacts with the sorbent such that at least aportion of the plurality of non-CO₂ acid gases are removed from theenvironment.